A PHASE 2, PLACEBO-CONTROLLED, RANDOMIZED, OBSERVER-BLINDED STUDY TO EVALUATE THE SAFETY, TOLERABILITY, AND IMMUNOGENICITY OF TWO 3-DOSE REGIMENS OF A CLOSTRIDIUM DIFFICILE VACCINE IN HEALTHY ADULTS AGED 65 TO 85 YEARS
Phase: PHASE2
Conditions: Clostridium Difficile Associated Disease
Sponsor: Pfizer

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A PHASE 2, PLACEBO-CONTROLLED, RANDOMIZED, OBSERVER-BLINDED 
STUDY TO EVALUATE THE SAFETY, TOLERABILITY, AND 
IMMUNOGENICITY OF TWO 3-DOSE REGIMENS OF A CLOSTRIDIUM 
DIFFICILE VACCINE IN HEALTHY ADULTS AGED 65 TO 85 YEARS
Compound:
PF-06425090
Compound Name:
Clostridium difficile vaccine
US IND Number:
15150
Protocol Number:
B5091009
Phase:
2

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Document History
Document
Version Date
Summary of Changes and Rationale.
Amendment 2
04-April-2017
Added an optional additional blood draw of 
20 mL to 50 mL of whole blood at Visit 12 for 
subjects enrolled in the extension stage (month 
regimen) at designated sites. Several changes 
were made throughout this protocol amendment 
to adjust blood volumes collected at Visit 12 and 
to modify language in order to allow whole blood 
sample testing for exploratory purposes.
Additional exploratory analyses will be 
performed on study samples to augment vaccine
assay development. Changes were made to the 
following sections:

Schedule of Activities (Schedule of 
Activities for Subjects Participating in the 
Extension Stage)

Study Procedures, Section 6.3.4 (Visit 12: 
Month 6 After Vaccination 4 [±30 Days])

Assessments, Section 7.2
(Immunogenicity)

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Amendment 1
11-April-2016
Added an extension stage to the study: This will 
include subjects in both dosing regimens who 
received the first 3 doses of C difficile vaccine 
(not placebo). Subjects in the extension stage will 
receive a fourth dose of C difficile at the same 
antigen dose level (100 µg or 200 µg) as they 
received previously or placebo, approximately 1 
year after their third dose. The subjects in the 
extension stage will be followed for a further 3 
years (visits approximately every 6 months) to 
assess antibody persistence.
Added objectives and endpoints for the extension 
stage.
Added eligibility criteria for the extension stage.
Added an unblinding step for subjects in the 
original portion and extension stages of the study.
Added a schedule of activities and corresponding 
text in the Procedures section for the extension 
stage.
Adverse event language was updated to reflect 
the reporting requirements for the extension stage 
(including the collection of research-related 
injuries [RRIs]).
Added an additional interim analysis for when 
Month 7 data are available from approximately 
the first 700 continuing subjects. The objective of 
the second analysis is to inform internal program-
level decision making.
Original protocol
01-April-2015
Not applicable (N/A)
This amendment incorporates all revisions to date, including amendments made at the 
request of country health authorities, institutional review boards/ethics committees 
IRBs/ECs), etc.

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TABLE OF CONTENTS
LIST OF TABLES.....................................................................................................................8
APPENDICES ...........................................................................................................................9
PROTOCOL SUMMARY.......................................................................................................10
SCHEDULE OF ACTIVITIES................................................................................................15
1. INTRODUCTION ...............................................................................................................22
1.1. Mechanism of Action/Indication.............................................................................22
1.2. Background and Rationale ......................................................................................22
1.2.1. Clostridium difficile Disease Background..................................................22
1.2.2. Clostridium difficile Vaccine Development Rationale...............................22
1.2.3. Clostridium difficile Vaccine Candidate.....................................................23
2. STUDY OBJECTIVES AND ENDPOINTS.......................................................................25
2.1. Objectives................................................................................................................25
2.1.1. Primary Immunogenicity Objectives..........................................................25
2.1.2. Primary Safety Objective............................................................................26
2.1.3. Secondary Immunogenicity Objectives......................................................26
2.1.4. Secondary Safety Objective........................................................................26
2.1.5. 
...............................................................................26
2.2. Endpoints.................................................................................................................27
2.2.1. Primary Immunogenicity Endpoints...........................................................27
2.2.2. Primary Safety Endpoints...........................................................................27
2.2.3. Secondary Immunogenicity Endpoints.......................................................27
2.2.4. Secondary Safety Endpoints.......................................................................29
2.2.5. 
..................................................................................30
3. STUDY DESIGN.................................................................................................................30
3.1. Approximate Number of Subjects...........................................................................30
3.2. Approximate Duration of Subject Participation......................................................30
3.3. Approximate Duration of the Study........................................................................31
3.4. Vaccine Groups and Enrollment .............................................................................31
4. SUBJECT SELECTION......................................................................................................32
4.1. Inclusion Criteria.....................................................................................................32
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4.2. Exclusion Criteria....................................................................................................32
4.3. Inclusion Criteria for the Extension Stage ..............................................................34
4.4. Exclusion Criteria for the Extension Stage .............................................................35
4.5. Criteria for Temporarily Delaying Vaccine Administration...................................36
4.6. Lifestyle Guidelines ................................................................................................37
4.7. Sponsor’s Qualified Medical Personnel..................................................................38
5. STUDY TREATMENTS.....................................................................................................38
5.1. Allocation to Investigational Product......................................................................38
5.2. Blinding of Site Personnel.......................................................................................39
5.3. Blinding of the Sponsor...........................................................................................39
5.4. Breaking the Blind ..................................................................................................39
5.5. Vaccine Supplies.....................................................................................................40
5.5.1. Formulation and Packaging........................................................................40
5.5.1.1. Clostridium difficile Vaccine ....................................................40
5.5.1.2. Placebo ......................................................................................40
5.5.2. Preparation and Dispensing........................................................................40
5.5.3. Administration............................................................................................41
5.5.4. Compliance.................................................................................................41
5.6. Investigational Product Storage ..............................................................................41
5.7. Investigational Product Accountability...................................................................42
5.7.1. Destruction of Investigational Product Supplies ........................................42
5.8. Concomitant Medication(s).....................................................................................43
5.8.1. Recording Concomitant Vaccinations and Medications.............................43
5.8.2. Prohibited Concomitant Vaccinations, Medications, and Blood 
Products............................................................................................................43
5.8.3. Permitted Concomitant Medications ..........................................................44
6. STUDY PROCEDURES .....................................................................................................44
6.1. Day 1, 8, and 30 Regimen.......................................................................................44
6.1.1. Visit 1: Day 1 - Vaccination 1 ....................................................................44
6.1.2. Visit 2: Day 8 - Vaccination 2 (7 to 11 Days After Visit 1).......................46
6.1.3. Visit 3: Day 15 (7 to 11 Days After Visit 2)...............................................48
6.1.4. Visit 4: Day 30 - Vaccination 3 (20 to 26 Days After Visit 2)...................49

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6.1.5. Visit 5: Day 37 (7 to 11 Days After Visit 4)...............................................51
6.1.6. Visit 6: Month 2 (25 to 35 Days After Visit 4)...........................................52
6.1.7. Visit 7: Month 4 (80 to 100 Days After Visit 4).........................................52
6.1.8. Visit 8: Month 7 (165 to 195 Days After Visit 4).......................................53
6.1.9. Visit 9: Month 13 (340 to 380 Days After Visit 4).....................................54
6.2. Month 0, 1, and 6 Regimen.....................................................................................54
6.2.1. Visit 1: Day 1 - Vaccination 1 ....................................................................54
6.2.2. Visit 2: Day 30 - Vaccination 2 (25 to 35 Days After Visit 1)...................56
6.2.3. Visit 3: Day 37 (7 to 11 Days After Visit 2)...............................................57
6.2.4. Visit 4: Month 2 (25 to 35 Days After Visit 2)...........................................59
6.2.5. Visit 5: Month 6 - Vaccination 3 (135 to 165 Days After Visit 2).............59
6.2.6. Visit 6: Day 187 (7 to 11 Days After Visit 5).............................................61
6.2.7. Visit 7: Month 7 (25 to 35 Days After Visit 5)...........................................62
6.2.8. Visit 8: Month 12 (165 to 195 Days After Visit 5).....................................62
6.2.9. Visit 9: Month 18 (340 to 380 Days After Visit 5).....................................63
6.3. Extension Stage.......................................................................................................63
6.3.1. Visit 9: Vaccination 4 (340 to 380 Days After Vaccination 3) ..................63
6.3.2. Visit 10: Day 8 After Vaccination 4 (7 to 11 Days After Visit 9)..............65
6.3.3. Visit 11: Day 30 After Vaccination 4 (25 to 35 Days After Visit 9)..........66
6.3.4. Visit 12: Month 6 After Vaccination 4 (±30 Days)....................................67
6.3.5. Visit 13: Month 12 After Vaccination 4 (±30 Days)..................................67
6.3.6. Visit 14: Month 18 After Vaccination 4 (±30 Days)..................................68
6.3.7. Visit 15: Month 24 After Vaccination 4 (±30 Days)..................................68
6.3.8. Visit 16: Month 30 After Vaccination 4 (±30 Days)..................................68
6.3.9. Visit 17: Month 36 After Vaccination 4 (±30 Days)..................................69
6.4. Severe Reaction Assessment and Unscheduled Visits............................................69
6.5. Subject Withdrawal.................................................................................................70
7. ASSESSMENTS..................................................................................................................71
7.1. Safety.......................................................................................................................71
7.1.1. Electronic Diary..........................................................................................72
7.1.1.1. Grading Scales...........................................................................72
7.1.1.2. Local Reactions.........................................................................72

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7.1.1.3. Systemic Events ........................................................................73
7.1.1.4. Fever..........................................................................................74
7.1.2. Triggered Safety Data Review Rule...........................................................75
7.2. Immunogenicity ......................................................................................................76
7.3. 
..................................................................................................77
8. ADVERSE EVENT REPORTING......................................................................................77
8.1. Adverse Events........................................................................................................77
8.2. Reporting Period .....................................................................................................77
8.3. Definition of an Adverse Event...............................................................................78
8.4. Medication Errors....................................................................................................79
8.5. Abnormal Test Findings..........................................................................................80
8.6. Serious Adverse Events...........................................................................................80
8.6.1. Protocol-Specified Serious Adverse Events...............................................81
8.6.2. Potential Cases of Drug-Induced Liver Injury............................................81
8.7. Research-Related Injury..........................................................................................82
8.8. Hospitalization ........................................................................................................83
8.9. Severity Assessment................................................................................................84
8.10. Causality Assessment............................................................................................84
8.11. Exposure During Pregnancy..................................................................................85
8.12. Occupational Exposure .........................................................................................86
8.13. Withdrawal Due to Adverse Events (See Also the Section on Subject 
Withdrawal)...............................................................................................................86
8.14. Eliciting Adverse Event Information ....................................................................86
8.15. Reporting Requirements........................................................................................86
8.15.1. Serious Adverse Event Reporting Requirements .....................................86
8.15.2. Nonserious Adverse Event Reporting Requirements ...............................87
8.15.3. Sponsor’s Reporting Requirements to Regulatory Authorities ................87
9. DATA ANALYSIS/STATISTICAL METHODS...............................................................87
9.1. Sample Size Determination.....................................................................................88
9.2. Immunogenicity Analysis .......................................................................................90
9.2.1. Immunogenicity Analysis Population.........................................................90
9.2.2. Analysis of the Primary Immunogenicity Endpoint...................................90
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9.2.3. Analysis of the Secondary and Other Immunogenicity Endpoints.............90
9.3. Safety Analysis........................................................................................................91
9.4. Analysis Timing ......................................................................................................91
9.4.1. First Interim Analysis .................................................................................92
9.4.2. Second Interim Analysis.............................................................................92
9.4.3. Third Interim Analysis................................................................................92
9.4.4. Fourth Interim Analysis..............................................................................92
9.4.5. Analysis for the Primary Clinical Study Report.........................................92
9.4.6. Analysis During the Extension Stage .........................................................93
9.5. Data Monitoring Committee ...................................................................................93
10. QUALITY CONTROL AND QUALITY ASSURANCE.................................................93
11. DATA HANDLING AND RECORD KEEPING .............................................................94
11.1. Case Report Forms/Electronic Data Record .........................................................94
11.2. Record Retention...................................................................................................94
12. ETHICS..............................................................................................................................95
12.1. Institutional Review Board/Ethics Committee......................................................95
12.2. Ethical Conduct of the Study ................................................................................95
12.3. Subject Information and Consent..........................................................................95
12.4. Subject Recruitment ..............................................................................................96
12.5. Reporting of Safety Issues and Serious Breaches of the Protocol or ICH 
GCP ...........................................................................................................................96
13. DEFINITION OF END OF TRIAL...................................................................................96
14. SPONSOR DISCONTINUATION CRITERIA ................................................................96
15. PUBLICATION OF STUDY RESULTS..........................................................................97
15.1. Communication of Results by Pfizer ....................................................................97
15.2. Publications by Investigators ................................................................................98
16. REFERENCES ..................................................................................................................99
LIST OF TABLES
Table 1.
Vaccine Groups and Number of Subjects per Group and per Dose Regimen......31
Table 2.
Local Reaction Grading Scale ..............................................................................73
Table 3.
Systemic Event Grading Scale..............................................................................74

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Table 4.
Scale for Fever......................................................................................................74
Table 5.
Triggered Safety Data Review Rule Criteria for Grade 3 Injection Site 
Erythema After the Specified Dose Number in the Specified Vaccination 
Regimen................................................................................................................75
Table 6.
Minimum Detectable Proportion of Subjects With the Specified Antibody 
Levels (Primary Immunogenicity Endpoint)........................................................88
Table 7.
Precision of the 2-Sided 95% Confidence Interval for Neutralizing 
Antibody Mean Concentration (in Log10) ...........................................................89
Table 8.
Probability of Observing at Least 1 AE by Assumed True Event Rates 
With Different Sample Sizes ................................................................................89
APPENDICES
Appendix 1. Abbreviations....................................................................................................102

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PROTOCOL SUMMARY
Background and Rationale
Clostridium difficile (C difficile), a gram-positive, anaerobic, spore-forming bacillus, is a 
major cause of antibiotic-associated nosocomial diarrhea in the developed world. In the last 
decade, the numbers and severity of C difficile iatrogenic outbreaks in hospitals and nursing 
homes have increased. Key factors in this escalation include emergence of hypervirulent 
pathogenic strains such as BI/NAP1/027, increased use of antibiotics, and increased exposure 
to spores in healthcare facilities.
Two large clostridial toxins, C difficile toxin A (TcdA) and C difficile toxin B (TcdB), are the 
principal virulence factors of C difficile–associated disease (CDAD) and both cause severe 
inflammation in the bowel. A causal relationship between toxin-producing clostridia and 
antibiotic-associated pseudomembranous colitis was first established in 1978. The clinical 
presentations of CDAD range from asymptomatic colonization to severe diarrhea, and may 
progress to pseudomembranous colitis, toxic megacolon, intestinal perforation, and death. 
Although most patients experiencing a first episode of CDAD respond well to standard 
antibiotic treatment, approximately 15% to 25% of the patients suffer from at least 1 
recurrence. However, the risk of subsequent episodes is 33% to 65% if the patient has 
already experienced a first recurrence. Fecal transplantation has been shown to be rather 
effective in the treatment of multiple recurrences of CDAD. However, regulatory oversight 
of this method remains in development and, therefore, a highly effective noncumbersome 
treatment for complicated CDAD does not exist.
Adults above the age of 65 years are at increased risk for CDAD, particularly in the 
healthcare setting, and the likelihood of severe infection among adults similarly increases 
with age. It is anticipated that the C difficile vaccine will ultimately be evaluated for efficacy 
and safety in an older target population, whose ability to mount an immune response may be 
impaired: This may influence the selection of an optimal vaccination regimen to ensure 
short- and long-term protection.
To date, there is no vaccine available to prevent CDAD. Investigational vaccines are in 
clinical development, but none is licensed. Pfizer’s vaccine candidate consists of a mixture of 
genetically modified C difficile toxoids A and B that are further chemically inactivated to 
eliminate residual cytotoxicity but retain native antigenic structure. In preclinical 
experiments, Pfizer’s vaccine candidate was studied either alone or in combination with an 
adjuvant. In the hamster disease model, all vaccine formulations demonstrated a survival 
benefit, providing at least 90% protection from a lethal challenge with C difficile spores in 
the immunized hamsters. In nonhuman primates (NHPs), all of Pfizer’s toxoid vaccine 
formulations tested induced robust neutralizing antitoxin antibody responses to both TcdA 
and TcdB.
Pfizer’s C difficile vaccine candidate was assessed in a Phase 1 study (B5091001), conducted 
in the United States, where 192 healthy adults aged 50 to 85 years were enrolled. This 
first-in-human (FIH) study was a dose-escalating, placebo-controlled, randomized, 
observer-blinded trial to evaluate the safety, tolerability, and immunogenicity of C difficile

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vaccine, as a 3-dose regimen at Months 0, 1, and 6. Three antigen dose levels (50, 100, and 
200 µg) of the vaccine candidate were assessed either alone or in combination with 
aluminum hydroxide.
The analysis of safety demonstrated that both formulations and all 3 dose levels were 
generally well tolerated. Local reactions were predominantly mild or moderate and 
comprised mostly injection site pain. No actual severe or Grade 4 local reactions were 
reported. In the 65- to 85-year age cohort, at the 200-µg dose level, local reactions tended to 
occur more frequently in the toxoid-alone compared to the aluminum hydroxide–containing
dose groups after all 3 doses. After all 3 doses, the frequency and severity of local reactions 
did not increase with increasing dose level or number of doses for any of the dose groups. 
Systemic events were predominantly mild to moderate and comprised mostly headache and 
fatigue. There was no evidence of increased frequency of systemic events with increasing 
dose level or number of doses for any of the dose groups.
The analysis of immunogenicity demonstrated a limited antibody response after Dose 1, but 
after Dose 2 there were marked increases in antibody titers against both toxin A and toxin B, 
which were generally maximal 7 days after Dose 2 and stable 1 month after Dose 2. Seven 
days after Dose 3, a substantial booster response was evident, which was slightly more 
marked again 1 month after Dose 3. Overall, robust antitoxin neutralizing responses were 
elicited by both formulations, although there was a trend for greater responses in recipients of 
the toxoid-alone formulation. With respect to the dose level, because of the limited sample 
size, 95% confidence intervals (CIs) were often overlapping for all 3 dose levels, indicating 
that it was not possible to discern a clear dose response.
In light of these results, a Phase 2 Study (B5091003) was designed to evaluate the safety, 
tolerability, and immunogenicity of the 100- and 200-g antigen dose levels of the toxoid-
alone formulation in a 3-dose regimen administered at Days 1, 8, and 30 in healthy adults 
50 to 85 years of age. Vaccinations in this study were stopped following the occurrence of 
7 cases of Grade 3 injection site erythema among 162 subjects receiving the second 
vaccination at Day 8. Among the recipients of the active vaccine, this corresponded to an 
incidence of 5.0% (95% CI 2.1% to 10.1%). There were no accompanying severe systemic 
symptoms in the subjects, there was no report that the redness impacted their daily activities,
and all local reactions fully resolved.
The local reactogenicity observed with the toxoid-alone formulation may have been due to 
free toxoid interacting with the elicited immune response. Since adsorption onto aluminum 
has been shown to bind and slowly release the vaccine constituents from the injection site, 
and reflecting the potential need for rapid induction of immune response and prolonged 
duration of protection, Study B5091009 will evaluate the safety, tolerability, and 
immunogenicity of 2 antigen dose levels of the aluminum hydroxide–containing vaccine 
(ie, 100 µg and 200 g) in 2 different dosing regimens: Days 1, 8, and 30 or Months 0, 1, and 
6. The primary immunogenicity endpoint will be assessed based upon a specified threshold 
estimate for each C difficile vaccine toxoid. The specified thresholds for each toxoid are 
considered to indicate protective titers.

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Since it will be important to provide vaccinated subjects with prolonged protection against 
CDAD, and since individuals targeted for vaccination may have diminished capacity to 
mount and maintain an immune response, subjects in the present study will be followed for 
4 years after their third vaccination. The purpose will be to assess antibody persistence and 
response to a fourth vaccination. Therefore, subjects in both dosing regimens who received 
the first 3 doses of C difficile vaccine (100 µg or 200 µg) will be enrolled into an extension 
stage. These subjects will receive a fourth dose of either C difficile vaccine at the same 
antigen dose level (100 µg or 200 µg) as they received previously or placebo, approximately 
1 year after their third dose. These subjects will be followed for a further 3 years to assess 
antibody persistence. Subjects originally randomized to placebo in either dosing regimen will 
not continue into the extension stage.
Primary Endpoints and Objectives
Primary Immunogenicity Objectives

To describe the immunogenicity of 2 antigen dose levels (100 µg and 200 µg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (Days 1, 8, and 30) 
to healthy adults aged 65 to 85 years, as measured by C difficile toxin A– and toxin B–
specific neutralizing antibody levels at Day 37 (7 days after Dose 3).

To describe the immunogenicity of 2 antigen dose levels (100 µg and 200 µg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (Months 0, 1, and 6) 
to healthy adults aged 65 to 85 years, as measured by C difficile toxin A– and toxin B–
specific neutralizing antibody levels at Month 7 (1 month after Dose 3).
Primary Safety Objective

To assess the safety and tolerability of 2 antigen dose levels (100 µg and 200 µg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (either Days 1, 8, 
and 30 or Months 0, 1, and 6) to healthy adults aged 65 to 85 years, by measuring local 
reactions and systemic events reported on subjects’ electronic diaries (e-diaries), adverse 
events (AEs), and serious AEs (SAEs).
Primary Immunogenicity Endpoints
At Day 37 (7 days after Dose 3 for subjects receiving the Day 1, 8, and 30 regimen) and at 
Month 7 (1 month after Dose 3 for subjects receiving the Month 0, 1, and 6 regimen), the 
proportions of subjects in each vaccine group with the following:

Toxin A–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin A;

Toxin B–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin B; and 

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
Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization units/mL) 
the specified threshold for toxin A and the specified threshold for toxin B, respectively.
Primary Safety Endpoints

Numbers and proportions of subjects reporting local reactions (pain, erythema, and 
induration) and their severity, as self-reported on e-diaries for up to 14 days following 
Vaccinations 1, 2, and 3.

Numbers and proportions of subjects reporting systemic events (fever, vomiting, 
diarrhea, headache, fatigue, new or worsening muscle pain, and new or worsening joint 
pain) and their severity, as self-reported on e-diaries for up to 14 days following 
Vaccinations 1, 2, and 3.

Numbers and proportions of subjects reporting AEs from the first vaccination up to 
28 days after the third vaccination categorized according to the Medical Dictionary for 
Regulatory Activities (MedDRA). 

Numbers and proportions of subjects reporting SAEs from the first vaccination until 
6 months after the third vaccination, categorized according to MedDRA.
Study Design
This is a Phase 2, placebo-controlled, randomized, observer-blinded study to assess the 
safety, tolerability, and immunogenicity of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of aluminum hydroxide–containing C difficile vaccine administered as a 3-dose 
regimen: either at Days 1, 8, and 30 or Months 0, 1, and 6. Approximately 854 subjects will 
be enrolled in the study. Subjects will be randomly assigned in parallel in a 3:3:1 ratio to 
receive C difficile vaccine (100 μg or 200 μg total toxoid) or placebo (saline) in 1 of the 2 
dosing regimens.
Subjects in both dosing regimens who received the first 3 doses of C difficile vaccine (100 μg 
or 200 μg) will be enrolled into an extension stage. These subjects will receive a fourth dose 
of either C difficile vaccine at the same antigen dose level (100 μg or 200 μg) as they 
received previously or placebo, approximately 1 year after their third dose. These subjects 
will be followed for a further 3 years (visits approximately every 6 months) to assess 
antibody persistence. Subjects originally randomized to placebo in either dosing regimen will 
not continue into the extension stage.
Investigational Products
 
 
 
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The placebo will consist of a sterile normal saline solution for injection (0.9% sodium 
chloride injection, in a 0.5-mL dose).
Statistical Method
The statistical analysis of the study results will be descriptive in nature. An estimation 
approach will be used to assess the safety and immunogenicity objectives in the study. 
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SCHEDULE OF ACTIVITIES
The schedule of activities table provides an overview of the protocol visits and procedures. Refer to the Study Procedures and 
Assessments sections of the protocol for detailed information on each procedure and assessment required for compliance with the 
protocol.
The investigator may schedule visits (unplanned visits) in addition to those listed on the schedule of activities, in order to conduct 
evaluations or assessments required to protect the well-being of the subject.
Schedule of Activities for Subjects Assigned to Receive 3 Doses (Days 1, 8, and 30)
Visit ID
Visit 1
Visit 2
Visit 3
Visit 4
Visit 5
Visit 6
Visit 7
Visit 8
Visit 9a
Visit Description
Vax 1
Day 1
Vax 2
Day 8
Day 15
Vax 3
Day 30
Day 37
Month 2
Month 4
Month 7
Month 13
(for 
subjects 
not 
entering 
the 
extension 
stage)
Visit Window (days)
1
7-11
Days After 
Visit 1
7-11
Days After 
Visit 2
20-26
Days After 
Visit 2
7-11 Days 
After 
Visit 4
25-35 Days 
After 
Visit 4
80-100
Days After 
Visit 4
165-195 
Days After 
Visit 4
340-380 
Days After 
Visit 4
Informed consent
X
Demography
X
Medical history 
X
Record nonstudy vaccinations and concomitant 
medications
X
X
X
X
X
X
Physical examination, including vital signs
X
Oral temperature
X
X
X
Confirm eligibility
X
X
X
X
X
X
X
X
Review temporary delay criteria
X
X
X

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Schedule of Activities for Subjects Assigned to Receive 3 Doses (Days 1, 8, and 30)
Visit ID
Visit 1
Visit 2
Visit 3
Visit 4
Visit 5
Visit 6
Visit 7
Visit 8
Visit 9a
Visit Description
Vax 1
Day 1
Vax 2
Day 8
Day 15
Vax 3
Day 30
Day 37
Month 2
Month 4
Month 7
Month 13
(for 
subjects 
not 
entering 
the 
extension 
stage)
Visit Window (days)
1
7-11
Days After 
Visit 1
7-11
Days After 
Visit 2
20-26
Days After 
Visit 2
7-11 Days 
After 
Visit 4
25-35 Days 
After 
Visit 4
80-100
Days After 
Visit 4
165-195 
Days After 
Visit 4
340-380 
Days After 
Visit 4
Randomization
X
Blood draw for immunogenicity assessment
X
X
X
X
X
X
X
X
X
Approximate total blood volume drawn per visitb
20 mL
20 mL
20 mL
20 mL
20 mL
20 mL
+
40 mLc
20 mL
20 mL
20 mL
Vaccination
X
X
X
Postvaccination observation (30 minutes) and 
acute AE assessment
X
X
X
Dispense e-diary, measuring device, and 
thermometer 
X
Targeted physical examination, including vital 
signs if appropriate
X
X
X
X
X
X
X
X
Review e-diary datad
Collect e-diary
X
Record AEs
X
X
X
X
X
X
Xe
Xe
Xe

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Schedule of Activities for Subjects Assigned to Receive 3 Doses (Days 1, 8, and 30)
Visit ID
Visit 1
Visit 2
Visit 3
Visit 4
Visit 5
Visit 6
Visit 7
Visit 8
Visit 9a
Visit Description
Vax 1
Day 1
Vax 2
Day 8
Day 15
Vax 3
Day 30
Day 37
Month 2
Month 4
Month 7
Month 13
(for 
subjects 
not 
entering 
the 
extension 
stage)
Visit Window (days)
1
7-11
Days After 
Visit 1
7-11
Days After 
Visit 2
20-26
Days After 
Visit 2
7-11 Days 
After 
Visit 4
25-35 Days 
After 
Visit 4
80-100
Days After 
Visit 4
165-195 
Days After 
Visit 4
340-380 
Days After 
Visit 4
Record SAEs 
X
X
X
X
X
X
X
X
Record newly diagnosed chronic medical 
conditions
X
X
a.
For subjects entering the extension stage, refer to the Schedule of Activities for Subjects Participating in the Extension Stage.
b.
Minimum of 10 mL and up to 20 mL blood will be collected (prior to vaccination if at a vaccination visit).
c.
An additional blood sample of approximately 40 mL will be collected for use in development of serologic assays and reagents.
d.
E-diary data review is ongoing during subject e-diary data-entry periods (7 days after the first vaccination and 14 days after the second and third 
vaccinations) via an Internet-based portal.
e.
Any AEs occurring up to 48 hours after each blood draw must be recorded on the CRF.
Abbreviations: CRF = case report form; e-diary = electronic diary; Vax = vaccination.

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Schedule of Activities for Subjects Assigned to Receive 3 Doses (Months 0, 1, and 6)
Visit ID
Visit 1
Visit 2
Visit 3
Visit 4
Visit 5
Visit 6
Visit 7
Visit 8
Visit 9a
Visit Description
Vax 1
Day 1
Month 0
Vax 2
Day 30
Month 1
Day 37
Month 2
Vax 3
Month 6
Day 187
Month 7
Month 12
Month 18
(for 
subjects 
not 
entering 
the 
extension 
stage)
Visit Window (days)
1
25-35
Days After 
Visit 1
7-11
Days After 
Visit 2
25-35 Days 
After 
Visit 2
135-165 
Days After 
Visit 2
7-11
Days After 
Visit 5
25-35 Days 
After 
Visit 5
165-195 
Days After 
Visit 5
340-380 
Days After 
Visit 5
Informed consent
X
Demography
X
Medical history 
X
Record nonstudy vaccinations and concomitant 
medications
X
X
X
X
X
X
X
Physical examination, including vital signs
X
Oral temperature
X
X
X
Confirm eligibility
X
X
X
X
X
X
X
X
Review temporary delay criteria
X
X
X
Randomization
X
Blood draw for immunogenicity assessment
X
X
X
X
X
X
X
X
X
Approximate total blood volume drawn per visitb
20 mL
20 mL
20 mL
20 mL
+
40 mLc
20 mL
20 mL
20 mL
20 mL
20 mL
Vaccination
X
X
X
Postvaccination observation (30 minutes) and 
acute AE assessment
X
X
X
Dispense e-diary, measuring device, and 
thermometer 
X

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Schedule of Activities for Subjects Assigned to Receive 3 Doses (Months 0, 1, and 6)
Visit ID
Visit 1
Visit 2
Visit 3
Visit 4
Visit 5
Visit 6
Visit 7
Visit 8
Visit 9a
Visit Description
Vax 1
Day 1
Month 0
Vax 2
Day 30
Month 1
Day 37
Month 2
Vax 3
Month 6
Day 187
Month 7
Month 12
Month 18
(for 
subjects 
not 
entering 
the 
extension 
stage)
Visit Window (days)
1
25-35
Days After 
Visit 1
7-11
Days After 
Visit 2
25-35 Days 
After 
Visit 2
135-165 
Days After 
Visit 2
7-11
Days After 
Visit 5
25-35 Days 
After 
Visit 5
165-195 
Days After 
Visit 5
340-380 
Days After 
Visit 5
Targeted physical examination, including vital 
signs if appropriate
X
X
X
X
X
X
X
X
Review e-diary datad
Collect e-diary
X
Record AEs
X
X
X
X
X
X
X
Xe
Xe
Record SAEs 
X
X
X
X
X
X
X
X
Record newly diagnosed chronic medical 
conditions
X
a.
For subjects entering the extension stage, refer to the Schedule of Activities for Subjects Participating in the Extension Stage.
b.
Minimum of 10 mL and up to 20 mL blood will be collected (prior to vaccination if at a vaccination visit).
c.
An additional blood sample of approximately 40 mL will be collected for use in development of serologic assays and reagents.
d.
E-diary data review is ongoing during subject e-diary data-entry periods (14 days after each vaccination) via an Internet-based portal.
e.
Any AEs occurring up to 48 hours after each blood draw must be recorded on the CRF.
Abbreviations: CRF = case report form; e-diary = electronic diary; Vax = vaccination.

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Schedule of Activities for Subjects Participating in the Extension Stage
Visit ID
Visit 9
Visit 10
Visit 11
Visit 12
Visit 13
Visit 14
Visit 15
Visit 16
Visit 17
Visit Description
Vax 4
Day 8 After 
Vax 4
Day 30 
After Vax 4
Month 6 
After Vax 4
Month 12
After Vax 4
Month 18
After Vax 4
Month 24
After Vax 4
Month 30
After Vax 4
Month 36
After Vax 4
Visit Window (days)
340-380 
Days After 
Vax 3a
7-11
Days After 
Visit 9
25-35
Days After 
Visit 9
30 
Days
30 
Days
30 
Days
30 
Days
30 
Days
30 
Days
Informed consent
X
Record nonstudy vaccinations and concomitant 
medications
X
X
X
Targeted physical examination, including vital 
signs if appropriate
X
X
X
Oral temperature
X
Confirm eligibility
X
X
X
X
X
X
X
X
Informed consent for optional blood drawb
X
Review temporary delay criteria
X
Rerandomization
X
Blood draw for immunogenicity assessment
X
X
X
X
X
X
X
X
X
Approximate total blood volume drawn per visitc
20 mL
20 mL
20 mL
20 mL (up 
to 70 mLd)
20 mL
20 mL
20 mL
20 mL
20 mL
Vaccination
X
Postvaccination observation (30 minutes) and 
acute AE assessment
X
Dispense e-diary, measuring device, and 
thermometer 
X
Review e-diary datae
Collect e-diary
X

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Schedule of Activities for Subjects Participating in the Extension Stage
Visit ID
Visit 9
Visit 10
Visit 11
Visit 12
Visit 13
Visit 14
Visit 15
Visit 16
Visit 17
Visit Description
Vax 4
Day 8 After 
Vax 4
Day 30 
After Vax 4
Month 6 
After Vax 4
Month 12
After Vax 4
Month 18
After Vax 4
Month 24
After Vax 4
Month 30
After Vax 4
Month 36
After Vax 4
Visit Window (days)
340-380 
Days After 
Vax 3a
7-11
Days After 
Visit 9
25-35
Days After 
Visit 9
30 
Days
30 
Days
30 
Days
30 
Days
30 
Days
30 
Days
Record AEs
X
X
X
Xf
Xf
Xf
Xf
Xf
Xf
Record SAEs 
X
X
X
X
Record RRIs
Xg
Xg
Xg
Xg
Xg
Xg
a.
Visit 4 for subjects on the Day 1, 8, and 30 regimen and Visit 5 for subjects on the Month 0, 1, and 6 regimen.
b.
Applicable only at designated sites.
c.
Minimum of 10 mL and up to 20 mL blood will be collected (prior to vaccination if at the vaccination visit).
d.
Applicable only for subjects at designated sites who have given consent for this additional blood draw.
e.
E-diary data review is ongoing during subject e-diary data-entry periods (14 days after vaccination) via an Internet-based portal.
f.
Any AEs occurring up to 48 hours after the blood draw must be recorded on the CRF.
g.
Any RRIs occurring up to 48 hours after the blood draw must be recorded on the CRF.
Abbreviations: CRF = case report form; e-diary = electronic diary; RRI = research-related injury; Vax = vaccination.

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1. INTRODUCTION
1.1. Mechanism of Action/Indication
Pfizer’s Clostridium difficile (C difficile) vaccine is intended to prevent C difficile–associated 
disease (CDAD) in an at-risk population.
1.2. Background and Rationale
1.2.1. Clostridium difficile Disease Background
C difficile, a gram-positive, anaerobic, spore-forming bacillus, is a major cause of 
antibiotic-associated nosocomial diarrhea in the developed world.1,2,3,4 In the last decade, the 
numbers and severity of C difficile iatrogenic outbreaks in hospitals and nursing homes have 
increased.5 Key factors in this escalation include emergence of hypervirulent pathogenic 
strains such as BI/NAP1/027,6,7,8 increased use of antibiotics,9 and increased exposure to 
spores in healthcare facilities.10
Two large clostridial toxins, C difficile toxin A (TcdA) and C difficile toxin B (TcdB), are the 
principal virulence factors of CDAD and both cause severe inflammation in the bowel.11
A causal relationship between toxin-producing clostridia and antibiotic-associated 
pseudomembranous colitis was first established in 1978.12 The clinical presentations of 
CDAD range from asymptomatic colonization to severe diarrhea, and may progress to 
pseudomembranous colitis, toxic megacolon, intestinal perforation, and death.2,13 Although 
most patients experiencing a first episode of CDAD respond well to standard antibiotic 
treatment, approximately 15% to 25% of the patients suffer from at least 1 recurrence. 
However, the risk of subsequent episodes is 33% to 65% if the patient has already 
experienced a first recurrence.14,15  Fecal transplantation has been shown to be rather 
effective in the treatment of multiple recurrences of CDAD.16,17 However, regulatory 
oversight of this method remains in development18 and, therefore, a highly effective 
noncumbersome treatment for complicated CDAD does not exist.
1.2.2. Clostridium difficile Vaccine Development Rationale
Humoral immune responses to C difficile toxins play a significant role in preventing a more 
severe outcome or a recurrence of the disease in humans. Several clinical studies suggest a 
correlation between high serum concentrations (as measured by enzyme-linked 
immunosorbent assay [ELISA]) of antitoxin A immunoglobulin G (IgG) and protection from 
CDAD.19,20,21 Active immunization with inactivated toxins and passive immunization with 
antitoxin antibodies have been demonstrated to protect animals from lethal challenge.22,23,24
In addition, hamsters treated with vancomycin alone have a higher mortality rate compared to 
those treated with vancomycin plus antibody, indicating that vaccination may provide an 
advantage over antibiotics.22 Furthermore, proof of concept that antibodies to both toxins are 
effective in prevention of CDAD comes from a Phase 2 study, where a combination of 
2 monoclonal antibodies (mAbs) against toxin A and toxin B was tested in subjects who had 
already had 1 episode of CDAD.25 All subjects (N=101) who received the mAbs and were 
protected from recurrence had approximately 10 to 100 g of mAbs/mL in their sera. The 
7 subjects who had a recurrence also exhibited similar antibody levels; however, it is not 
known whether the failure in protection of these 7 subjects was due to exposure to 

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C difficile–produced toxin variants not recognized by the mAbs used in the study. In the 
placebo group, all but 1 of the 25 subjects who suffered recurrences had less than 1 g/mL of 
neutralizing antibodies in their sera.25 Taken together, the Phase 2 study efficacy data 
suggested that antitoxin A and antitoxin B neutralizing mAb levels above a threshold of 
10 g/mL were associated with protection against CDAD. However, the minimum level of 
antibody required for protection could not be discerned.25 Together, these findings provide a 
clinical rationale for the development of an anti–C difficile toxin A and B vaccine to prevent 
primary CDAD and associated disease recurrence.
Adults above the age of 65 years are at increased risk for CDAD, particularly in the 
healthcare setting, and the likelihood of severe infection among adults similarly increases 
with age.14,26,27,28,29,30 It is anticipated that the C difficile vaccine will ultimately be evaluated 
for efficacy and safety in an older target population, whose ability to mount an immune 
response may be impaired: This may influence the selection of an optimal vaccination 
regimen to ensure short- and long-term protection.
To date, there is no vaccine available to prevent CDAD. Investigational vaccines are in 
clinical development, but none is licensed. A vaccine consisting of formalin-inactivated 
TcdA and TcdB is currently in Phase 3 clinical development.31,32 Also, a Phase 2 study with 
a recombinant protein vaccine candidate incorporating truncated portions of TcdA and TcdB 
has recently started.33
1.2.3. Clostridium difficile Vaccine Candidate
Pfizer’s vaccine candidate consists of a mixture of genetically modified C difficile toxoids A 
and B that are further chemically inactivated to eliminate residual cytotoxicity but retain 
native antigenic structure.34 In preclinical experiments, Pfizer’s vaccine candidate was 
studied either alone or in combination with an adjuvant. In the hamster disease model, all 
vaccine formulations demonstrated a survival benefit, providing at least 90% protection from 
a lethal challenge with C difficile spores in the immunized hamsters. In nonhuman primates 
(NHPs), all of Pfizer’s toxoid vaccine formulations tested induced robust neutralizing 
antitoxin antibody responses to both TcdA and TcdB.
Pfizer’s C difficile vaccine candidate was assessed in a Phase 1 study (B5091001), conducted 
in the United States, where 192 healthy adults aged 50 to 85 years were enrolled. This 
first-in-human (FIH) study was a dose-escalating, placebo-controlled, randomized, 
observer-blinded trial to evaluate the safety, tolerability, and immunogenicity of C difficile
vaccine, as a 3-dose regimen at Months 0, 1, and 6. Three antigen dose levels (50, 100, and 
200 µg) of the vaccine candidate were assessed either alone or in combination with 
aluminum hydroxide.
The analysis of safety demonstrated that both formulations and all 3 dose levels were 
generally well tolerated. Local reactions were predominantly mild or moderate and 
comprised mostly injection site pain. No actual severe or Grade 4 local reactions were 
reported. In the 65- to 85-year age cohort, at the 200-µg dose level, local reactions tended to 
occur more frequently in the toxoid-alone compared to the aluminum hydroxide–containing
dose groups (41.7%, 50.0%, and 80.0% versus 16.7%, 18.2%, and 11.1%, for any local 

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reaction after each of the 3 doses, respectively). After all 3 doses, the frequency and severity 
of local reactions did not increase with increasing dose level or number of doses for any of 
the dose groups. Systemic events were predominantly mild to moderate and comprised 
mostly headache and fatigue. There was no evidence of increased frequency of systemic 
events with increasing dose level or number of doses for any of the dose groups.
The analysis of immunogenicity demonstrated a limited antibody response after Dose 1, but 
after Dose 2 there were marked increases in antibody titers against both toxin A and toxin B, 
which were generally maximal 7 days after Dose 2 and stable 1 month after Dose 2. Seven 
days after Dose 3, a substantial booster response was evident, which was slightly more 
marked again 1 month after Dose 3. Overall, robust antitoxin neutralizing responses were 
elicited by both formulations, although there was a trend for greater responses in recipients of 
the toxoid-alone formulation. For example, 1 month after Dose 3 in the 65- to 85-year age 
cohort, the geometric mean fold rises (GMFRs) from baseline (before Dose 1) in toxin A–
specific neutralizing antibody titers ranged from 131.41 to 254.77 in the toxoid-alone dose 
groups and from 42.73 to 80.66 in the aluminum hydroxide–containing dose groups. The 
corresponding ranges for toxin B–specific neutralizing antibody titers were from 2953.12 to
4922.80 and from 136.12 to 484.14, respectively. With respect to the dose level, because of 
the limited sample size, 95% confidence intervals (CIs) were often overlapping for all 3 dose 
levels, indicating that it was not possible to discern a clear dose response. Preclinical data 
generated by Pfizer in rhesus macaques support the use of a 3-dose regimen of C difficile
vaccine, administered with or without aluminum hydroxide at Weeks 0, 2, and 4. 
Furthermore, in a rabbit toxicology study, a 4-dose regimen of C difficile vaccine (up to 
400-g dose levels) given on Days 1, 8, 22, and 36 did not demonstrate adverse toxicological 
findings, and resulted in an increase in antitoxin A– and antitoxin B–neutralizing antibody 
titers, confirming the anticipated immunologic response by the animals to the administered 
immunogen.35
These preclinical and toxicology data, as well as the encouraging immune response observed 
after 3 doses in the B5091001 Phase 1 study, supported the Phase 2 Study
 
 
 
 
 
 
 
 
 
 
 
 
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. These specified thresholds were derived from a 
Phase 2 efficacy study demonstrating that passive administration of 2 mAbs against toxin A 
and toxin B were associated with protection against CDAD. In addition to showing efficacy 
of antitoxin mAbs against recurrent CDAD, the Phase 2 efficacy study also suggested that 
antitoxin A– and antitoxin B–neutralizing mAb levels above a threshold of 10 g/mL were 
associated with protection against CDAD recurrence.25 Although the minimum level of 
antibody required for protection could not be discerned, the data derived from this study did 
allow estimation of the 50% neutralization titer elicited by the Pfizer vaccine candidate that 
would predict similar vaccine efficacy against CDAD 
 
 
 
 
(2) the inhibitory mAb concentration that neutralizes 50% 
of the toxins (IC50) had been published.22
Since it will be important to provide vaccinated subjects with prolonged protection against 
CDAD, and since individuals targeted for vaccination may have diminished capacity to 
mount and maintain an immune response, subjects in the present study will be followed for 4 
years after their third vaccination. The purpose will be to assess antibody persistence and 
response to a fourth vaccination. Therefore, subjects in both dosing regimens who received 
the first 3 doses of C difficile vaccine (100 μg or 200 μg) will be enrolled into an extension 
stage and will be rerandomized in a 1:1 ratio to receive C difficile vaccine or placebo. These 
subjects will receive a fourth dose of either C difficile vaccine at the same antigen dose level 
(100 μg or 200 μg) as they received previously or placebo, approximately 1 year after their 
third dose. These subjects will be followed for a further 3 years to assess antibody 
persistence. Subjects originally randomized to placebo in either dosing regimen will not 
continue into the extension stage.
Complete information for this vaccine may be found in the single reference safety document 
(SRSD), which for this study is the investigator’s brochure (IB).
2. STUDY OBJECTIVES AND ENDPOINTS
2.1. Objectives
2.1.1. Primary Immunogenicity Objectives

To describe the immunogenicity of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (Days 1, 8, and 30) 
to healthy adults aged 65 to 85 years, as measured by C difficile toxin A– and toxin B–
specific neutralizing antibody levels at Day 37 (7 days after Dose 3).

To describe the immunogenicity of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (Months 0, 1, and 6) 
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to healthy adults aged 65 to 85 years, as measured by C difficile toxin A– and toxin B–
specific neutralizing antibody levels at Month 7 (1 month after Dose 3).
2.1.2. Primary Safety Objective 

To assess the safety and tolerability of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of C difficile vaccine when administered as a 3-dose regimen (either Days 1, 8, 
and 30 or Months 0, 1, and 6) to healthy adults aged 65 to 85 years, by measuring local 
reactions and systemic events reported on subjects’ electronic diaries (e-diaries), adverse 
events (AEs), and serious AEs (SAEs).
2.1.3. Secondary Immunogenicity Objectives

To describe the immunogenicity of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of C difficile vaccine when administered in a 3-dose regimen (either Days 1, 8, 
and 30 or Months 0, 1, and 6) to healthy adults aged 65 to 85 years, as measured by 
C difficile toxin A– and toxin B–specific neutralizing antibody levels at multiple time 
points following vaccination.

To describe the kinetics of the immune response in healthy adults aged 65 to 85 years for 
up to 12 months following the administration of 3 doses of C difficile vaccine.

To describe the immunogenicity of a fourth dose of C difficile vaccine as measured by 
C difficile toxin A– and toxin B–specific neutralizing antibody levels at multiple time 
points following vaccination.

To describe the kinetics of the immune response in healthy adults aged 65 to 85 years for 
up to 36 months following the administration of a fourth dose of C difficile vaccine.
2.1.4. Secondary Safety Objective 

To assess the safety and tolerability of a fourth dose of C difficile vaccine by measuring 
local reactions and systemic events reported on subjects’ e-diaries, AEs, and SAEs.
2.1.5.
 

 
 

 

 
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2.2. Endpoints
2.2.1. Primary Immunogenicity Endpoints
At Day 37 (7 days after Dose 3 for subjects receiving the Day 1, 8, and 30 regimen) and at 
Month 7 (1 month after Dose 3 for subjects receiving the Month 0, 1, and 6 regimen), the 
proportions of subjects in each vaccine group with the following:

Toxin A–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin A;

Toxin B–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin B; and 

Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization units/mL) 
the specified threshold for toxin A and the specified threshold for toxin B, respectively.
2.2.2. Primary Safety Endpoints

Numbers and proportions of subjects reporting local reactions (pain, erythema, and 
induration) and their severity, as self-reported on e-diaries for up to 14 days following 
Vaccinations 1, 2, and 3.

Numbers and proportions of subjects reporting systemic events (fever, vomiting, 
diarrhea, headache, fatigue, new or worsening muscle pain, and new or worsening joint 
pain) and their severity, as self-reported on e-diaries for up to 14 days following  
Vaccinations 1, 2, and 3.

Numbers and proportions of subjects reporting AEs from the first vaccination up to 
28 days after the third vaccination categorized according to the Medical Dictionary for 
Regulatory Activities (MedDRA). 

Numbers and proportions of subjects reporting SAEs from the first vaccination until 
6 months after the third dose of vaccine, categorized according to MedDRA.
2.2.3. Secondary Immunogenicity Endpoints
At Day 37 (7 days after Dose 3 for subjects receiving the Day 1, 8, and 30 regimen) and at 
Month 7 (1 month after Dose 3 for subjects receiving the Month 0, 1, and 6 regimen):

Toxin A– and toxin B–specific neutralizing antibody levels, expressed as geometric mean 
concentrations (GMCs) (neutralization units/mL).

GMFRs from baseline (before Dose 1) in

Toxin A–specific and

Toxin B–specific neutralizing antibody levels (neutralization units/mL).

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
Proportions of subjects in each vaccine group with 4-fold, 8-fold, 16-fold, and 
32-fold rises from baseline in

Toxin A–specific;

Toxin B–specific; and

Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization 
units/mL).
For subjects receiving the Day 1, 8, and 30 regimen, on Day 1 (immediately before Dose 1), 
Day 8 (immediately before Dose 2), Day 15 (7 days after Dose 2), Day 30 (immediately 
before Dose 3), and Months 2 (1 month after Dose 3), 4 (3 months after Dose 3), 7 (6 months 
after Dose 3), and 13 (12 months after Dose 3); and for subjects receiving the Month 0, 1,
and 6 regimen, on Day 1 (immediately before Dose 1), Day 30 (immediately before Dose 2), 
Day 37 (7 days after Dose 2), Months 2 (1 month after Dose 2) and 6 (immediately before 
Dose 3), Day 187 (7 days after Dose 3), and Months 12 (6 months after Dose 3) and 18 
(12 months after Dose 3):

Proportions of subjects in each vaccine group with

Toxin A–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin A;

Toxin B–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin B; and

Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization 
units/mL) ≥ the specified threshold for toxin A and the specified threshold for 
toxin B, respectively (these parameters will also be assessed at baseline).

Toxin A– and toxin B–specific neutralizing antibody levels, expressed as GMCs 
(neutralization units/mL).

GMFRs from baseline in

Toxin A–specific; and

Toxin B–specific neutralizing antibody levels (neutralization units/mL).

Proportions of subjects in each vaccine group with 4-fold, 8-fold, 16-fold, and 
32-fold rises from baseline in

Toxin A–specific;

Toxin B–specific; and

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
Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization 
units/mL).
For subjects enrolled in the extension stage; at the fourth dose (immediately before the fourth
dose), at Days 8 and 30 after the fourth dose, and at Months 6, 12, 18, 24, 30, and 36 after the 
fourth dose:

Proportions of subjects in each vaccine group with

Toxin A–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin A;

Toxin B–specific neutralizing antibody level (neutralization units/mL) the specified 
threshold for toxin B; and 

Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization 
units/mL) ≥ the specified threshold for toxin A and the specified threshold for 
toxin B, respectively (these parameters will also be assessed at baseline).

Toxin A– and toxin B–specific neutralizing antibody levels, expressed as GMCs 
(neutralization units/mL).

GMFRs from baseline in

Toxin A–specific; and

Toxin B–specific neutralizing antibody levels (neutralization units/mL).

Proportions of subjects in each vaccine group with 4-fold, 8-fold, 16-fold, and 
32-fold rises from baseline in

Toxin A–specific;

Toxin B–specific; and

Both toxin A– and toxin B–specific neutralizing antibody levels (neutralization 
units/mL).
2.2.4. Secondary Safety Endpoints

Numbers and proportions of subjects reporting local reactions (pain, erythema, and 
induration) and their severity, as self-reported on e-diaries for to 14 days following the 
fourth vaccination.

Numbers and proportions of subjects reporting systemic events (fever, vomiting, 
diarrhea, headache, fatigue, new or worsening muscle pain, and new or worsening joint 

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pain) and their severity, as self-reported on e-diaries for 14 days following the fourth
vaccination.

Numbers and proportions of subjects reporting AEs from the time of the fourth
vaccination up to 28 days after the fourth vaccination categorized according to the 
Medical Dictionary for Regulatory Activities (MedDRA). 

Numbers and proportions of subjects reporting SAEs from the time of the fourth
vaccination until 6 months after the fourth dose of vaccine, categorized according to 
MedDRA.
2.2.5.

 
3. STUDY DESIGN
This is a Phase 2, placebo-controlled, randomized, observer-blinded study to assess the 
safety, tolerability, and immunogenicity of 2 antigen dose levels (100 μg and 200 μg total 
toxoid) of aluminum hydroxide–containing C difficile vaccine administered as a 3-dose 
regimen: either at Days 1, 8, and 30 or Months 0, 1, and 6.
Subjects will be assigned to 1 of the 2 dosing regimens and then randomly assigned in 
parallel in a 3:3:1 ratio to receive C difficile vaccine (100 μg or 200 μg total toxoid) or 
placebo (saline) (see Table 1).
Subjects in both dosing regimens who received the first 3 doses of C difficile vaccine (100 μg 
or 200 μg) will be enrolled into an extension stage. These subjects will receive a fourth dose 
of either C difficile vaccine at the same antigen dose level (100 μg or 200 μg) as they 
received previously or placebo, approximately 1 year after their third dose. These subjects 
will be followed for a further 3 years (visits approximately every 6 months) to assess 
antibody persistence. Subjects originally randomized to placebo in either dosing regimen will 
not continue into the extension stage.
3.1. Approximate Number of Subjects
Approximately 854 healthy adults, aged 65 to 85 years will be enrolled at approximately 15
sites in the United States. The number of subjects enrolled at each site may vary based on 
enrollment capabilities of each site.
Subjects withdrawn from the study after randomization will not be replaced, regardless of the 
reason for withdrawal.
3.2. Approximate Duration of Subject Participation
For the original portion of the study, subjects will be followed for 12 months after receipt of 
their third vaccination. Therefore, subjects assigned to the Day 1, 8, and 30 regimen will 
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participate for approximately 13 months and those assigned to the Month 0, 1, and 6 regimen 
will participate for approximately 18 months.
Subjects who enter the extension stage will be followed for a further 36 months. Therefore,
subjects originally assigned to the Day 1, 8, and 30 regimen may participate for 
approximately 4 years and those assigned to the Month 0, 1, and 6 regimen will participate 
for approximately 4.5 years.
3.3. Approximate Duration of the Study
The original portion of the study will be completed in approximately 21 months. The total 
duration of the study (including the extension stage) will be approximately 5 years.
The end of the study is the last visit of the last subject as described in Section 13.
3.4. Vaccine Groups and Enrollment
For the original portion of the study, subjects will be assigned to 1 of 6 study groups listed in
Table 1. For each dosing regimen, approximately 366 subjects will receive the C difficile
vaccine (at either 100 or 200 µg total toxoid dose level) and approximately 61 subjects will 
receive placebo (saline).
Table 1.
Vaccine Groups and Number of Subjects per Group and per Dose Regimen
Vaccine 
Group
Vaccine Formulation Description
Dosing Regimen
Number of Subjects
1a
Aluminum hydroxide–containing C difficile 
vaccine (100-µg antigen dose)
Days 1,8,30 
183
2 a
Aluminum hydroxide–containing C difficile 
vaccine (200-µg antigen dose)
Days 1,8,30 
183
3
Placebo (saline)
Days 1,8,30
61
4 a
Aluminum hydroxide–containing C difficile 
vaccine (100-µg antigen dose)
Months 0,1,6
183
5 a
Aluminum hydroxide–containing C difficile 
vaccine (200-µg antigen dose)
Months 0,1,6
183
6
Placebo (saline)
Months 0,1,6
61
Total
854
a.
Subjects in these groups will be asked to enter the extension stage.
All eligible subjects who received 3 doses of C difficile vaccine (Groups 1, 2, 4, and 5) will 
be asked to enter the extension stage. Subjects enrolled in the extension stage will be 
rerandomized to receive a fourth dose of either C difficile vaccine at the same antigen dose 
level (100 µg or 200 µg) as they received previously or placebo.

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4. SUBJECT SELECTION
This study can fulfill its objectives only if appropriate subjects are enrolled.  The following 
eligibility criteria are designed to select subjects for whom participation in the study is 
considered appropriate.  All relevant medical and nonmedical conditions should be taken into 
consideration when deciding whether a particular subject is suitable for this protocol.
4.1. Inclusion Criteria
Subject eligibility should be reviewed and documented by an appropriately qualified member 
of the investigator’s study team before subjects are included in the study.
Subjects must meet all of the following inclusion criteria to be eligible for enrollment into the 
study:
1. Evidence of a personally signed and dated informed consent document (ICD) indicating 
that the subject has been informed of all pertinent aspects of the study.
2. Healthy male and female adults aged 65 to 85 years at enrollment (signing of the ICD) as 
determined by medical history, physical examination, and the clinical judgment of the 
investigator to be eligible for the study. Subjects with preexisting chronic medical 
conditions determined to be stable may be included.
3. Male subjects able to father children must agree to use a highly effective method of 
contraception from the time of informed consent through at least 28 days after the last 
dose of investigational product.
4. Female subjects who are not of childbearing potential (ie, meet at least 1 of the following 
criteria):

Have undergone hysterectomy and/or bilateral oophorectomy; 

Have medically confirmed ovarian failure; or 

Achieved postmenopausal status, defined as cessation of regular menses for at least 
12 consecutive months with no alternative pathological or physiological cause; status 
may be confirmed by having a serum follicle-stimulating hormone (FSH) level 
confirming the post menopausal state.
5. Availability for the entire duration of the study, and able to comply with scheduled visits, 
treatment plan, and study procedures, including completion of the e-diary.
6. Ability to be contacted by telephone during study participation.
4.2. Exclusion Criteria
Subjects with any of the following characteristics/conditions will not be included in the 
study:

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1. Previous administration of an investigational C difficile vaccine or C difficile mAb 
therapy.
2. Proven or suspected prior episode of CDAD. Please refer to the study reference manual 
(SRM) for additional guidance.
3. Unstable chronic medical condition or disease requiring significant change in therapy or 
hospitalization for worsening disease within 8 weeks before receipt of investigational 
product.
4. Serious chronic medical disorders, including metastatic malignancy, severe chronic 
obstructive pulmonary disease (COPD) requiring supplemental oxygen, end-stage renal 
disease with or without dialysis, clinically unstable cardiac disease, or any other disorder 
that in the investigator’s opinion precludes the subject from participating in the study.
5. Bleeding diathesis or condition associated with prolonged bleeding time that may 
contraindicate intramuscular injection, including subjects taking anticoagulant, 
antiplatelet, and/or antithrombotic agents except for low-dose daily aspirin 
(325 mg/day) within 30 days before enrollment until 1 month after the last vaccination.
6. Any contraindication to vaccination or vaccine components, including previous 
anaphylactic reaction to any vaccine or vaccine-related components.
7. Subjects with congenital or acquired immunodeficiency disorders, or subjects with 
rheumatologic disorders or other illnesses requiring chronic treatment with known 
immunosuppressant medications, including mAbs, within a year of enrollment or the use 
of systemic corticosteroids for 7 days within 90 days prior to enrollment through 
conclusion of the study (eg, 10 mg/day of prednisone for 7 days).
8. Active or treated leukemia or lymphoma or underlying bone marrow disorder such as 
myelodysplasia, myeloma, or myeloproliferative disorder, treated within the past year, or 
any history of bone marrow transplant.
9. Malignancy that required treatment with chemotherapy (including the use of adjunctive 
and hormonal therapy), immunotherapy, radiation therapy, or antineoplastic target 
therapies within the past 24 months.
10. Subjects who received oral or parenteral antibiotics within 1 month before enrollment. 
Topical antibiotics are allowed.
11. Receipt of blood products or immunoglobulins (including mAbs) within 6 months before 
enrollment through conclusion of the study.
12. Participation in other studies involving investigational drug(s) within 30 days prior to 
study entry up to and including Visit 12. Participation in purely observational studies is 
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13. Subjects who are investigational site staff members directly involved in the conduct of 
the study and their family members, site staff members otherwise supervised by the 
investigator, or subjects who are Pfizer employees directly involved in the conduct of the 
study.
14. Females of childbearing potential; males of child-fathering potential who are unwilling or 
unable to use a highly effective method of contraception as outlined in this protocol for at 
least 28 days after the last dose of investigational product.
15. Residence in a nursing home or other long-term care facility, or requirement for 
semiskilled nursing care or assisted living. An ambulatory subject who lives in an 
autonomous manner in a retirement home or village is eligible for the trial.
16. A known infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV), 
and/or hepatitis C virus (HCV).
17. Other severe acute or chronic medical or psychiatric condition or laboratory abnormality 
that may increase the risk associated with study participation or investigational product 
administration or may interfere with the interpretation of study results and, in the 
judgment of the investigator, would make the subject inappropriate for entry into this 
study.
4.3. Inclusion Criteria for the Extension Stage
Subject eligibility should be reviewed and documented by an appropriately qualified member 
of the investigator’s study team before subjects are included in the extension stage.
Subjects must meet all of the following inclusion criteria to be eligible to enter the extension 
stage:
1. Evidence of a personally signed and dated ICD addendum indicating that the subject has 
been informed of all pertinent aspects of the extension stage of the study.
2. Healthy male and female adults as determined by medical history, physical examination, 
and the clinical judgment of the investigator to be eligible for the study. Subjects with 
preexisting chronic medical conditions determined to be stable may be included.
3. Receipt of all 3 doses of C difficile vaccine (100-µg or 200-µg antigen dose level) in the 
original portion of the study.
4. Male subjects able to father children must agree to use a highly effective method of 
contraception from the time of informed consent through at least 28 days after the fourth
dose.
5. Female subjects who are not of childbearing potential (ie, meet at least 1 of the following 
criteria):

Have undergone hysterectomy and/or bilateral oophorectomy; 

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
Have medically confirmed ovarian failure; or 

Achieved postmenopausal status, defined as cessation of regular menses for at least 
12 consecutive months with no alternative pathological or physiological cause; status 
may be confirmed by having a serum FSH level confirming the postmenopausal state.
6. Availability for the entire duration of the study, and able to comply with scheduled visits, 
treatment plan, and study procedures, including completion of the e-diary. 
7. Ability to be contacted by telephone during study participation.
4.4. Exclusion Criteria for the Extension Stage
Subjects with any of the following characteristics/conditions will not be eligible to enter the 
extension stage:
1. Subjects originally randomized to placebo in either dosing regimen during the original 
portion of the study.
2. Subjects who have already completed Visit 9 prior to study unblinding.
3. Previous administration of a C difficile mAb therapy.
4. Proven or suspected prior episode of CDAD. Please refer to the SRM for additional 
guidance.
5. Unstable chronic medical condition or disease requiring significant change in therapy or 
hospitalization for worsening disease within 8 weeks before receipt of investigational 
product.
6. Serious chronic medical disorders, including metastatic malignancy, severe COPD 
requiring supplemental oxygen, end-stage renal disease with or without dialysis, 
clinically unstable cardiac disease, or any other disorder that in the investigator’s opinion 
precludes the subject from participating in the study.
7. Bleeding diathesis or condition associated with prolonged bleeding time that may 
contraindicate intramuscular injection, including subjects taking anticoagulant, 
antiplatelet, and/or antithrombotic agents except for low-dose daily aspirin 
(325 mg/day) within 30 days before until 1 month after the fourth dose.
8. Any contraindication to vaccination or vaccine components, including previous 
anaphylactic reaction to any vaccine or vaccine-related components.
9. Subjects with congenital or acquired immunodeficiency disorders, or subjects with 
rheumatologic disorders or other illnesses requiring chronic treatment with known 
immunosuppressant medications, including mAbs, within a year of enrollment or the use 
of systemic corticosteroids for 7 days within 90 days prior to enrollment through 
conclusion of the study (eg, 10 mg/day of prednisone for 7 days).

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10. Active or treated leukemia or lymphoma or underlying bone marrow disorder such as 
myelodysplasia, myeloma, or myeloproliferative disorder, treated within the past year, or 
any history of bone marrow transplant.
11. Malignancy that required treatment with chemotherapy (including the use of adjunctive 
and hormonal therapy), immunotherapy, radiation therapy, or antineoplastic target 
therapies within the past 24 months.
12. Subjects who received oral or parenteral antibiotics within 1 month before the fourth
dose. Topical antibiotics are allowed.
13. Receipt of blood products or immunoglobulins (including mAbs) within 6 months before 
enrollment through conclusion of the study.
14. Participation in other studies involving investigational drug(s) within 30 days prior to 
study entry up to and including Visit 12. Participation in purely observational studies is 
acceptable.
15. Subjects who are investigational site staff members directly involved in the conduct of 
the study and their family members, site staff members otherwise supervised by the 
investigator, or subjects who are Pfizer employees directly involved in the conduct of the 
study.
16. Females of childbearing potential; males of child-fathering potential who are unwilling or 
unable to use a highly effective method of contraception as outlined in this protocol for at 
least 28 days after the fourth dose.
17. Residence in a nursing home or other long-term care facility, or requirement for 
semiskilled nursing care or assisted living. An ambulatory subject who lives in an 
autonomous manner in a retirement home or village is eligible for the trial.
18. A known infection with HIV, HBV, and/or HCV.
19. Other severe acute or chronic medical or psychiatric condition or laboratory abnormality 
that may increase the risk associated with study participation or investigational product 
administration or may interfere with the interpretation of study results and, in the 
judgment of the investigator, would make the subject inappropriate for vaccination with 
the fourth dose.
4.5. Criteria for Temporarily Delaying Vaccine Administration
The following conditions are temporary or self-limiting and a subject may be vaccinated 
once the condition(s) has/have resolved and no other exclusion criteria are met:
1. Current febrile illness (oral temperature of 100.4F [38.0C]) or other acute illness 
within 48 hours prior to investigational product administration.

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2. Subject has received licensed nonlive influenza vaccine within the previous 14 days or 
any other vaccine within the previous 28 days before investigational product 
administration.
3. Subject has received systemic antibiotic therapy for an acute illness within 72 hours 
before the study vaccination.
4. If systemic corticosteroids have been administered short term (<7 days) for treatment of 
an acute illness, investigational product administration should be delayed until systemic 
corticosteroid use has been discontinued for at least 1 month. Inhaled, 
intra-articular/intrabursal, or topical corticosteroids are permitted.
If a subject meets any delay criteria for vaccination, all study procedures, including blood 
sample collection relating to that visit, should be delayed until the day of vaccination.
4.6. Lifestyle Guidelines
All male subjects who are able to father children and are sexually active must agree to use a 
highly effective method of contraception consistently and correctly from the signing of the 
ICD to 28 days after the last dose of investigational product. The investigator or his or her 
designee, in consultation with the subject, will select an appropriate method of contraception 
for the individual subject and his partner from the permitted list of contraception methods
(see below), and instruct the subject in its consistent and correct use. Subjects need to affirm 
that they meet the criteria for correct use of at least 1 of the selected methods of 
contraception. The investigator or his or her designee, at each study visit, will confirm and 
document consistent and correct use and document such conversation in the subject’s chart.
In addition, the investigator or his or her designee will instruct the subject to call immediately 
if the selected contraception method is discontinued or if pregnancy in his partner is known 
or suspected.
Highly effective methods of contraception are those that, alone or in combination, result in a 
failure rate of less than 1% per year when used consistently and correctly (ie, perfect use) and 
include the following:
1. Established use of oral, inserted, injected, implanted, or transdermal hormonal methods 
of contraception is allowed provided the subject’s partner remains on the same treatment 
throughout the entire study and has been using that hormonal contraceptive for an 
adequate period of time to ensure effectiveness.
2. Correctly placed copper-containing intrauterine device (IUD). 
3. Male condom or female condom used WITH a spermicide (ie, foam, gel, film, cream, 
suppository).
4. Male sterilization with absence of sperm in the postvasectomy ejaculate.

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5. Bilateral tubal ligation, bilateral salpingectomy, or bilateral tubal occlusive procedure 
(provided that occlusion has been confirmed in accordance with the device’s label).
6. Female partner who meets the criteria for nonchildbearing potential, as described below:

Have undergone a documented hysterectomy and/or bilateral oophorectomy;

Have medically confirmed ovarian failure; or 

Achieved postmenopausal status, defined as follows: cessation of regular menses for 
at least 12 consecutive months with no alternative pathological or physiological 
cause.
4.7. Sponsor’s Qualified Medical Personnel
The contact information for the sponsor's appropriately qualified medical personnel for the 
trial is documented in the SRM.
To facilitate access to appropriately qualified medical personnel on study-related medical 
questions or problems, subjects are provided with a contact card. The contact card contains, 
at a minimum, protocol and investigational compound identifiers, subject study number, 
contact information for the investigational site, and contact details for a contact center in the 
event that the investigational site staff cannot be reached to provide advice on a medical 
question or problem originating from another healthcare professional not involved in the 
subject’s participation in the study. The contact number can also be used by investigational 
staff if they are seeking advice on medical questions or problems; however, it should only be 
used in the event that the established communication pathways between the investigational 
site and the study team are not available. It is therefore intended to augment, but not replace, 
the established communication pathways between the investigational site and study team for 
advice on medical questions or problems that may arise during the study. The contact center 
number is not intended for use by the subject directly, and if a subject calls that number, he 
or she will be directed back to the investigational site.
5. STUDY TREATMENTS
For the purposes of this study, and per International Council for Harmonisation (ICH)
guidelines, investigational product is defined as a pharmaceutical form of an active 
ingredient or placebo being tested or used as a reference in a clinical trial, including a 
product with a marketing authorization when used or assembled (formulated or packaged) in 
a way different from the approved form, or when used for an unapproved indication, or when 
used to gain further information about an approved use (ICH E6 1.33).
5.1. Allocation to Investigational Product
Allocation of subjects to vaccine groups (at both entry to the study and entry in the extension 
stage) will proceed through the use of the interactive response technology (IRT) system that 
is accessible 24 hours a day, 365 days a year. Specific instructions for use of the IRT system 
will be provided under separate cover. The study site will obtain the subject’s randomization 

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number and container number (or equivalent) assignment from the IRT system. The 
randomization number and the date on which the randomization number was assigned will be 
recorded on the case report form (CRF). Once subject numbers, container numbers, and 
randomization numbers have been assigned, they cannot be reassigned.
5.2. Blinding of Site Personnel
In this observer-blinded study, the study staff dispensing and administering the vaccine will 
be unblinded, but all other site study personnel, including the principal investigator and the 
subject, will be blinded during the vaccination periods of the study.
Following the third interim analysis (when Month 7 [after the first dose] data are available 
from all continuing subjects), site personnel will be unblinded to vaccine assignments for the 
first 3 doses. This will enable site personnel to identify subjects eligible to enter the extension 
stage. During the extension stage, site personnel (excluding the study staff dispensing and 
administering the vaccine) will be blinded to vaccine allocation at the fourth dose until 
Visit 12 (6 months after the fourth dose).
The principal investigator will assign the responsibility of unblinded dispenser and unblinded 
administrator to persons who will not participate in the evaluation of any study subject.
More than 1 unblinded dispenser/administrator may be assigned per site. A member of the 
study site staff or clinic pharmacy should fulfill this role. Contact between the unblinded 
dispenser and study subjects should be kept to a minimum. The investigator, study 
coordinator, and any site staff other than the unblinded dispenser/administrator must not be 
allowed to know the investigational product assigned to any study subject and must not be 
allowed to see the investigational product container contents.
5.3. Blinding of the Sponsor
Those study team members who are involved in ensuring that protocol requirements for 
investigational product handling, allocation, and administration are fulfilled at the site will be 
unblinded for the duration of the study. All other study team members will remain blinded to 
vaccine assigned/received at Doses 1, 2, and 3 until the third interim analysis. Those study 
team members will also be blinded to vaccine assigned/received at Dose 4 until Visit 12 
(6 months after the fourth dose). Laboratory personnel performing the immunologic assays 
will remain blinded to vaccine assigned/received throughout the study. Certain sponsor 
personnel not involved in the conduct of the study will review unblinded data as defined in 
an internal review committee (IRC) charter per Pfizer standard operating procedures (SOPs).
In addition to the IRC members, an unblinded statistician, an unblinded physician, and an 
unblinded clinical scientist, who are not part of the study team, will be assigned to work with 
an external independent statistical center supporting the IRC and external data monitoring 
committee (E-DMC) review activities.
5.4. Breaking the Blind
At the initiation of the study, each study site will be instructed on the method for breaking the 
blind. The method will be either a manual or electronic process. Blinding codes should only 
be broken in emergency situations for reasons of subject safety. Whenever possible, the 

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investigator or subinvestigator consults with a member of the study team prior to breaking 
the blind. If this is not possible within the time frame required for breaking the blind, a 
member of the study team should be contacted as soon as possible afterwards. The breaking 
of the blind will be recorded in the appropriate section of the CRF.
At the appropriate time points for unblinding, site personnel will be able to access subject 
vaccine allocations for Doses 1, 2, and 3 and Dose 4 using the IRT system.
5.5. Vaccine Supplies
5.5.1. Formulation and Packaging
5.5.1.1. Clostridium difficile Vaccine
 
 
 
 
 
 
 
 
 
 
 
 
 
5.5.1.2. Placebo
The placebo will consist of a sterile normal saline solution for injection (0.9% sodium 
chloride injection, in a 0.5-mL dose) and will be provided by the sponsor to each study site. 
The placebo will be packaged within cartons labeled by Pfizer or its designee in accordance 
with current guidelines and applicable local and legal regulatory requirements. Blinded, 
sealed cartons will contain 1 single-use 2-mL vial of placebo. Refer to the DAI for details 
regarding the preparation and dispensing of placebo.
5.5.2. Preparation and Dispensing
See the dosage and administration instructions (DAI) for instructions on how to prepare the 
investigational product for administration.  Investigational product should be prepared and 
dispensed by an appropriately qualified and experienced unblinded member of the study 
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staff (eg, physician, nurse, physician’s assistant, practitioner, or pharmacist) as allowed by 
local, state, and institutional guidance.
5.5.3. Administration
All injections will be administered in the upper deltoid muscle, preferably of the 
nondominant arm, by the unblinded administrator. For subjects receiving vaccinations on 
the Day 1, 8, and 30 regimen, prior to the second and third vaccinations, appropriate blinded
site staff should evaluate the injection site for any ongoing reactions from the previous 
vaccination. If an injection site reaction is still present, investigational product should be 
administered by intramuscular injection in the upper deltoid muscle of the opposite arm.
Subjects on the Day 1, 8, and 30 regimen will receive 1 dose of C difficile vaccine/placebo at 
Visits 1 [Day 1], 2 [Day 8], and 4 [Day 30]). Subjects on the Month 0, 1, and 6 regimen will 
receive 1 dose of C difficile vaccine/placebo at Visits 1 [Day 1], 2 [Day 30], and 
5 [Month 6]). Subjects entering the extension stage will receive 1 dose of C difficile 
vaccine/placebo at Visit 9. Administration of investigational products should be performed 
by an appropriately qualified, Good Clinical Practice (GCP)-trained, and 
vaccine-experienced member of the study staff (eg, physician, nurse, physician’s assistant, 
pharmacist, or medical assistant) as allowed by local, state, and institutional guidance.
Standard vaccination practices must be observed and vaccine must not be injected into blood 
vessels. Appropriate medication and other supportive measures for management of an acute 
hypersensitivity reaction should be available in accordance with local guidelines for standard 
vaccination practices.
Investigational product administration details (ie, date of administration, randomization 
number, volume, route, and site of administration, and administration errors) will be recorded 
on the CRF. Investigational product accountability will be documented as per Section 5.7.
5.5.4. Compliance
All doses of investigational product will be administered by the appropriately designated 
unblinded study staff at the investigative site. The information will be recorded on source 
documents, on the investigational product accountability record, and in the CRF.
5.6. Investigational Product Storage
At each site, the investigator, or an approved representative, eg, pharmacist, will ensure that 
all investigational products are stored in a secured area with controlled access under required
storage conditions and in accordance with applicable regulatory requirements.
Investigational product should be stored in its original container and in accordance with the 
label. Storage conditions stated in the SRSD (ie, IB) will be superseded by the storage
conditions stated in the label.
 
 
CCI

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Site systems must be capable of measuring and documenting (for example, via a log), at a 
minimum, daily minimum and maximum temperatures for all site storage locations (as 
applicable, including frozen, refrigerated, and/or room-temperature products). This should 
be captured from the time of investigational product receipt throughout the study. Even for 
continuous monitoring systems, a log or site procedure that ensures active daily evaluation 
for excursions should be available. The operation of the temperature monitoring device and 
storage unit (for example, refrigerator), as applicable, should be regularly inspected to ensure 
it is maintained in working order.
Any excursions from the product label storage conditions should be reported upon discovery. 
The site should actively pursue options for returning the product to the storage conditions as 
described in the labeling, as soon as possible. Deviations from the storage requirements, 
including any actions taken, must be documented and reported to the sponsor.
Once an excursion is identified, the investigational product must be quarantined and not used 
until the sponsor provides documentation of permission to use the investigational product. It 
will not be considered a protocol deviation if the sponsor approves the use of the 
investigational product after the temperature excursion. Use of the investigational product 
prior to sponsor approval will be considered a protocol deviation. Specific details regarding 
information the site should report for each excursion will be provided to the site.
Guidance on temperature monitoring and procedures for the review of temperature deviations 
will be provided in the investigational product manual. Preparation instructions and stability 
information can be found in the DAI.
5.7. Investigational Product Accountability
The unblinded vaccine dispenser must maintain a complete and current accountability record, 
documenting the receipt, use, loss, or other disposition of the investigational product 
supplies.
Investigational product accountability applies to such products when they are required by the 
protocol and supplied (shipped) by the sponsor (including aluminum hydroxide diluent
specified for reconstitution).
5.7.1. Destruction of Investigational Product Supplies
The sponsor or designee will provide guidance on the destruction of unused investigational 
product (eg, at the site).  If destruction is authorized to take place at the study site, the 
investigator must ensure that the materials are destroyed in compliance with applicable 
environmental regulations, institutional policy, and any special instructions provided by 
Pfizer, and all destruction must be adequately documented.
CCI

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5.8. Concomitant Medication(s)
5.8.1. Recording Concomitant Vaccinations and Medications
Subjects will be asked to provide a history of the name and date for all vaccinations received 
from 6 months prior to enrollment until 1 month after the third vaccination (Visit 6 [Month 2] 
for subjects on the Day 1, 8, and 30 regimen and Visit 7 [Month 7] for subjects on the 
Month 0, 1, and 6 regimen) and this information will be recorded in the CRF.
Subjects entering the extension stage will be asked to provide a history of the name and date 
for all vaccinations received from 28 days prior to vaccination with the fourth dose until 
Visit 11 (Day 30 after the fourth dose), and this information will be recorded in the CRF.
Subjects will be asked to provide the start date, stop date, dose, unit, frequency, route, and 
indication for concomitant medications taken after signing the ICD until 1 month after the 
third vaccination (Visit 6 [Month 2] for subjects on the Day 1, 8, and 30 regimen and Visit 7 
[Month 7] for subjects on the Month 0, 1, and 6 regimen). Details of this information will be 
recorded in the CRF.
Subjects will be asked to provide the start date, stop date, dose, unit, frequency, route, and 
indication for concomitant medications taken from the time of vaccination with the fourth 
dose until Visit 11 (Day 30 after the fourth dose), and this information will be recorded in the 
CRF.
5.8.2. Prohibited Concomitant Vaccinations, Medications, and Blood Products
Unless considered medically necessary, no vaccines (licensed or investigational) other than 
study vaccine should be administered between signing the ICD and 1 month after the third 
vaccination (Visit 6 [Month 2] for subjects on the Day 1, 8, and 30 regimen and Visit 7 
[Month 7] for subjects on the Month 0, 1, and 6 regimen) or from the time of the fourth dose 
until Visit 11 (Day 30 after the fourth dose). Whenever possible, any nonstudy vaccine that is 
considered medically necessary should be administered at least 28 days after, or at least 
28 days prior to, administration of study vaccine. Note one exception: licensed nonlive 
influenza vaccine may be administered at least 14 days after, or at least 14 days prior to, the 
administration of study vaccine.
Immunosuppressive therapy is prohibited throughout the study; this includes chemotherapy 
agents or immune-modifying drugs, the latter including, but not limited to, disease-modifying 
antirheumatic drugs (DMARDs) such as methotrexate and the tumor necrosis factor 
antagonists, and immune response modifiers such as, but not limited to, ustekinumab. Use of 
systemic corticosteroids (10 mg/day of prednisone or equivalent) for 7 days is prohibited 
within 90 days prior to enrollment through the conclusion of the study.
Any blood products, including immunoglobulin, are also prohibited throughout the course of 
the study.
Prophylactic antipyretics and other pain medication to prevent symptoms associated with 
investigational product administration are not permitted. However, if a subject is taking a

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permitted medication (according to Section 5.8.3) for another condition, even if it may have 
antipyretic or pain-relieving properties, it should not be withheld prior to study vaccination.
An e-diary will be used to capture the use of antipyretics/pain medication to treat local 
reactions and systemic events for 7 or 14 days (dependent upon assigned vaccination 
regimen) following the first vaccination and 14 days following the second and third 
vaccinations as described in Section 7.1.1. For subjects entering the extension stage, the 
e-diary will capture the use of antipyretics/pain medication to treat local reactions and 
systemic events for 14 days after the fourth dose as described in Section 7.1.1.
5.8.3. Permitted Concomitant Medications
The use of antipyretics and other pain medication to treat symptoms associated with 
investigational product administration or ongoing conditions is permitted.
Medication other than that described as prohibited in Section 5.8.2 required for treatment of 
preexisting stable conditions is permitted.
Inhaled, topical, or localized injections of corticosteroids (eg, intra-articular or intrabursal 
administration) are permitted.
Low-dose aspirin (325 mg/day) is permitted when used as an antiplatelet agent.
6. STUDY PROCEDURES
The schedule of procedures is presented in the Schedule of Activities. In order for the study 
visits to be scheduled within the correct time frames, the day of the first vaccination should 
be considered Day 1.
Before enrollment and before any study-related procedures are performed, voluntary, written 
study-specific informed consent will be obtained from the subject. Each signature on the 
ICD must be personally dated by the signatory. The investigator or his or her designee will 
also sign the ICD. A copy of the signed and dated ICD must be given to the subject. The 
source data must reflect that the informed consent was obtained before participation in the 
study.
Before randomization, the investigator or a medically qualified designee will review the 
subject’s medical history and medications to ensure that the subject meets all of the inclusion 
criteria, none of the exclusion criteria, and none of the temporary delay criteria. The 
procedures required at each visit are detailed below.
6.1. Day 1, 8, and 30 Regimen
6.1.1. Visit 1: Day 1 - Vaccination 1

Obtain written informed consent prior to performing any protocol-required procedures.

Record the subject’s demography (including date of birth, sex, race, and ethnicity). 

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
Record medical history of significance, including the presence of chronic medical 
conditions and relevant surgical procedures.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Perform a physical examination evaluating any clinically significant abnormalities within 
the following body systems: general appearance, skin, head/eyes/ears/nose/throat, heart,
lungs, abdomen, extremities, neurological, and lymph nodes. Abnormal results must be 
recorded on source documents and the medical history CRF.

Perform vital sign measurements, including weight, height, sitting blood pressure, sitting 
pulse rate, and respiratory rate. When multiple measurements are taken for the same time 
point, the most out-of-range (eg, highest or lowest) value must be reported. (See the SRM 
for additional guidance.)

Measure and record the subject’s oral temperature.

Ensure that all inclusion criteria, none of the exclusion criteria, and none of the temporary 
delay criteria are met (Section 4).

An appropriate site staff member (may be blinded or unblinded) will use the IRT system 
to obtain the subject’s randomization number and investigational product blinded carton 
number. Unblinded site staff will prepare and administer investigational product. Please 
refer to the SRM under separate cover for further instruction on this process.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Investigational product will be administered by intramuscular injection, by the unblinded 
administrator, into the upper deltoid muscle, preferably of the nondominant arm.

The unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Issue a measuring device for measurement of local reactions, and a digital thermometer 
for recording of daily temperatures, and provide instructions on their use.

Issue a subject e-diary and provide instructions on its completion. Ask the subject to 
complete the e-diary from Day 1 to Day 7, with Day 1 being the day of vaccination.

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
Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 7 days following vaccination.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.1.2. Visit 2: Day 8 - Vaccination 2 (7 to 11 Days After Visit 1)

Review the subject’s e-diary data and follow up on any ongoing local reactions or 
systemic events.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

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
Measure and record the subject’s oral temperature.

Ensure that none of the temporary delay criteria are met (Section 4.5).

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

An appropriately qualified blinded member of the site staff should evaluate the previous 
injection site for any ongoing reactions from the previous vaccination. If there is no 
persisting injection site reaction from the previous vaccine administration, the 
investigational product should be administered by the unblinded administrator. If an 
injection site reaction is still present, investigational product should be administered by 
intramuscular injection in the upper deltoid muscle of the opposite arm by the unblinded
administrator.

The unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Remind the subject to complete the e-diary from Day 1 to Day 14, with Day 1 being the 
day of vaccination. Instruct the subject that e-diary entries for injection site symptoms 
should be in relation to the vaccination received that day and not to the previous 
vaccination. 

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F).

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 7 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

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
Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.1.3. Visit 3: Day 15 (7 to 11 Days After Visit 2)

Review the subject’s e-diary data and remind the subject to continue completing the 
e-diary up to Day 14.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following:

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

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
Any severe systemic event within 14 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.1.4. Visit 4: Day 30 - Vaccination 3 (20 to 26 Days After Visit 2)

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Review the subject’s e-diary data from Dose 2 and follow up on any ongoing local 
reactions or systemic events.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Measure and record the subject’s oral temperature.

Ensure that none of the temporary delay criteria are met (Section 4.5).

Collect a blood sample of approximately 20 mL for immunogenicity assessments 
(Section 7.2).

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
An appropriately qualified blinded member of the site staff should evaluate the previous 
injection site for any ongoing reactions from the previous vaccination. If there is no 
persisting injection site reaction from the previous vaccine administration, the 
investigational product should be administered by the unblinded administrator. If an 
injection site reaction is still present, investigational product should be administered by 
intramuscular injection in the upper deltoid muscle of the opposite arm by the unblinded
administrator.

The unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Remind the subject to complete the e-diary from Day 1 to Day 14, with Day 1 being the 
day of vaccination. Instruct the subject that e-diary entries for injection site symptoms 
should be in relation to the vaccination received that day and not to the previous 
vaccination.

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 14 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

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
Complete the CRF.
6.1.5. Visit 5: Day 37 (7 to 11 Days After Visit 4)

Review the subject’s e-diary data and remind the subject to continue completing the 
e-diary up to Day 14.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F).

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 14 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

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
Schedule an appointment for the subject to return for the next study visit. 

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.1.6. Visit 6: Month 2 (25 to 35 Days After Visit 4)

Review the subject’s e-diary data after Dose 3 and follow up on any ongoing local 
reactions or systemic events.

Collect the subject’s e-diary.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2). 
An additional blood sample of approximately 40 mL will be obtained.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.1.7. Visit 7: Month 4 (80 to 100 Days After Visit 4)

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

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
Based on clinical evaluation, determine whether any newly diagnosed chronic medical 
conditions or SAEs have occurred since the last study visit. Record and report findings 
as described in Section 8 and the Schedule of Activities. Review AEs that were ongoing 
from the previous visit and record their stop dates or confirm if they are still continuing.
In addition, any AEs occurring up to 48 hours after the blood draw will be recorded.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.1.8. Visit 8: Month 7 (165 to 195 Days After Visit 4)

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any newly diagnosed chronic medical 
conditions or SAEs have occurred since the last study visit. Record and report findings 
as described in Section 8 and the Schedule of Activities. Review AEs that were ongoing 
from the previous visit and record their stop dates or confirm if they are still continuing.
In addition, any AEs occurring up to 48 hours after the blood draw will be recorded.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.

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6.1.9. Visit 9: Month 13 (340 to 380 Days After Visit 4)

If study-level unblinding following the third interim analysis has already occurred 
(Section 5.3), use the IRT system to determine if the subject received C difficile vaccine 
(not placebo) at Doses 1, 2, and 3. If the subject received C difficile vaccine (at either 
antigen dose level of 100 µg or 200 µg), obtain consent for the subject to enter the 
extension stage (Section 6.3).
For subjects entering the extension stage, follow the procedures in Section 6.3.1
rather than the procedures listed below. For the subjects not entering the extension 
stage, complete the procedures below.

Review AEs that were ongoing from the previous visit and record their stop dates or 
confirm if they are still continuing. In addition, any AEs occurring up to 48 hours after 
the blood draw will be recorded.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. 

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Complete the source documents.

Complete the CRF.
6.2. Month 0, 1, and 6 Regimen
6.2.1. Visit 1: Day 1 - Vaccination 1

Obtain written informed consent prior to performing any protocol-required procedures.

Record the subject’s demography (including date of birth, sex, race, and ethnicity).

Record medical history of significance, including the presence of chronic conditions and 
relevant surgical procedures.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Perform a physical examination evaluating any clinically significant abnormalities within 
the following body systems: general appearance, skin, head/eyes/ears/nose/throat, heart, 
lungs, abdomen, extremities, neurological, and lymph nodes. Abnormal results must be 
recorded on source documents and the medical history CRF.

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
Perform vital sign measurements, including weight, height, sitting blood pressure, sitting 
pulse rate, and respiratory rate. When multiple measurements are taken for the same time 
point, the most out-of-range (eg, highest or lowest) value must be reported. (See the 
SRM for guidance.)

Measure and record the subject’s oral temperature.

Ensure that all inclusion criteria, none of the exclusion criteria, and none of the temporary 
delay criteria are met (Section 4).

An appropriate site staff member (may be blinded or unblinded) will use the IRT system 
to obtain the subject’s randomization number and investigational product blinded carton 
number. Unblinded site staff will prepare and administer investigational product. Please 
refer to the SRM under separate cover for further instruction on this process.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Investigational product will be administered by intramuscular injection, by the unblinded 
administrator, into the upper deltoid muscle, preferably of the nondominant arm.

The unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Issue a measuring device for measurement of local reactions, and a digital thermometer 
for recording of daily temperatures, and provide instructions on their use.

Issue a subject e-diary and provide instructions on its completion. Ask the subject to 
complete the e-diary from Day 1 to Day 14, with Day 1 being the day of vaccination.

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

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
Any severe systemic event within 14 days following vaccination.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.2.2. Visit 2: Day 30 - Vaccination 2 (25 to 35 Days After Visit 1)

Review the subject’s e-diary data and follow up on any ongoing local reactions or 
systemic events.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Measure and record the subject’s oral temperature.

Ensure that none of the temporary delay criteria are met (Section 4.5).

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Investigational product will be administered by intramuscular injection, by the unblinded 
administrator, into the upper deltoid muscle, preferably of the nondominant arm.

The unblinded vaccine administrator updates the investigational product accountability 
records.

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
Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Remind the subject to complete the e-diary from Day 1 to Day 14, with Day 1 being the 
day of vaccination. Instruct the subject that e-diary entries for injection site symptoms 
should be in relation to the vaccination received that day and not to the previous 
vaccination. 

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 7 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.2.3. Visit 3: Day 37 (7 to 11 Days After Visit 2)

Review the subject’s e-diary data and remind subjects to continue completing the e-diary 
up to Day 14.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

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
Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if he or she 
experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 14 days following vaccination to determine if an 
unscheduled visit is required.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.

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6.2.4. Visit 4: Month 2 (25 to 35 Days After Visit 2)

Review the subject’s e-diary data after Dose 2 and follow up on any ongoing local 
reactions or systemic events.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2). 
An additional blood sample of approximately 40 mL will be obtained.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.2.5. Visit 5: Month 6 - Vaccination 3 (135 to 165 Days After Visit 2)

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 

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self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Measure and record the subject’s oral temperature.

Ensure that none of the temporary delay criteria are met (Section 4.5).

Collect a blood sample of approximately 20 mL for immunogenicity assessments 
(Section 7.2).

Investigational product will be administered by intramuscular injection, by the unblinded 
administrator, into the upper deltoid muscle, preferably of the nondominant arm. The 
unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Remind the subject to complete the e-diary from Day 1 to Day 14, with Day 1 being the 
day of vaccination. Instruct the subject that e-diary entries for injection site symptoms 
should be in relation to the vaccination received that day and not to the previous 
vaccination.

Ask the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F).

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 7 days following vaccination.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

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
Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.2.6. Visit 6: Day 187 (7 to 11 Days After Visit 5)

Review the subject’s e-diary data and remind subjects to continue completing the e-diary 
up to Day 14.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.

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6.2.7. Visit 7: Month 7 (25 to 35 Days After Visit 5)

Review the subject’s e-diary data after Dose 3 and follow up on any ongoing local 
reactions or systemic events.

Collect the subject’s e-diary.

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. 

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.2.8. Visit 8: Month 12 (165 to 195 Days After Visit 5)

Ensure that the subject continues to be eligible for the study and does not meet any of the 
withdrawal criteria as described in Section 6.5.

Based on clinical evaluation, determine whether any newly diagnosed chronic medical 
conditions or SAEs have occurred since the last study visit. Record and report findings 
as described in Section 8 and the Schedule of Activities. Review AEs that were ongoing 
from the previous visit and record their stop dates or confirm if they are still continuing.
In addition, any AEs occurring up to 48 hours after the blood draw will be recorded.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

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
Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.2.9. Visit 9: Month 18 (340 to 380 Days After Visit 5)

If study-level unblinding following the third interim analysis has already occurred 
(Section 5.3), use the IRT system to determine if the subject received C difficile vaccine 
(not placebo) at Doses 1, 2, and 3. If the subject received C difficile vaccine (at either 
antigen dose level of 100 µg or 200 µg), obtain consent for the subject to enter the 
extension stage (Section 6.3).
For subjects entering the extension stage, follow the procedures in Section 6.3.1
rather than the procedures listed below. For the subjects not entering the extension 
stage, complete the procedures below.

Review AEs that were ongoing from the previous visit and record their stop dates or 
confirm if they are still continuing. In addition, any AEs occurring up to 48 hours after 
the blood draw will be recorded.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. 

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Complete the source documents.

Complete the CRF.
6.3. Extension Stage
6.3.1. Visit 9: Vaccination 4 (340 to 380 Days After Vaccination 3)

Obtain written informed consent for participation in the extension stage prior to 
performing any of the extension-stage procedures.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

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
Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Measure and record the subject’s oral temperature.

Ensure that all inclusion criteria for the extension stage, none of the exclusion criteria for 
the extension stage, and none of the temporary delay criteria are met (Section 4).

An appropriate site staff member (may be blinded or unblinded) will use the IRT system 
to obtain the subject’s randomization number for the extension stage and investigational 
product blinded carton number. Unblinded site staff will prepare and administer 
investigational product. Please refer to the SRM under separate cover for further 
instruction on this process.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Investigational product will be administered by intramuscular injection, by the unblinded 
administrator, into the upper deltoid muscle, preferably of the nondominant arm.

The unblinded vaccine administrator updates the investigational product accountability 
records.

Blinded study staff will observe the subject for at least 30 minutes after investigational 
product administration for any acute reactions. Any AEs noted during the observation 
period should be recorded as immediate AEs.

Record and report AEs and SAEs (relative to the time of vaccination) as described in 
Section 8 and the Schedule of Activities.

Issue a measuring device for measurement of local reactions, and a digital thermometer 
for recording of daily temperatures, and provide instructions on their use.

Issue a subject e-diary and provide instructions on its completion. Ask the subject to 
complete the e-diary from Day 1 to Day 14, with Day 1 being the day of vaccination. Ask 
the subject to contact the site staff or investigator immediately, to determine if an 
unscheduled visit is required, if he or she experiences any of the following: 

Fever 39.0C (102.1F). 

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

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
Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain.

Any severe systemic event within 14 days following vaccination.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.3.2. Visit 10: Day 8 After Vaccination 4 (7 to 11 Days After Visit 9)

Review the subject’s e-diary data and remind subjects to continue completing the e-diary 
up to Day 14.

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Ask the subject to contact the site staff or investigator immediately if he or she 
experiences any of the following: 

Fever 39.0C (102.1F).

Redness or swelling at the injection site measuring greater than 10 cm 
(21 measuring device units).

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
Any peeling/scaling or blackening of the skin at the injection site.

Severe arm pain. 

Any severe systemic event within 14 days following vaccination to determine if an 
unscheduled visit is required.

If any new, late-onset reaction occurs from the previous vaccination.

If any ongoing reaction from the previous vaccination worsens.

Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Remind the subject to bring the completed e-diary to the next visit.

Complete the source documents.

Complete the CRF.
6.3.3. Visit 11: Day 30 After Vaccination 4 (25 to 35 Days After Visit 9)

Review the subject’s e-diary data after Dose 4 and follow up on any ongoing local 
reactions or systemic events.

Collect the subject’s e-diary.

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Based on clinical evaluation, determine whether any AEs or SAEs have occurred since 
the last study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing.

Record nonstudy vaccines and concomitant medications as described in Section 5.8.1.

Conduct a targeted physical examination, including vital signs if appropriate, evaluating 
any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the last visit. Abnormal results, including 
those that indicate worsening of medical history conditions, must be recorded on source 
documents and the AE CRF (with recording of time relative to vaccination) as 
appropriate.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

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
Ask the subject to contact the site staff or investigator immediately if any significant 
illness or hospitalization occurs.

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.3.4. Visit 12: Month 6 After Vaccination 4 (±30 Days)

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Based on clinical evaluation, determine whether any SAEs have occurred since the last 
study visit. Record and report findings as described in Section 8 and the 
Schedule of Activities. Review AEs that were ongoing from the previous visit and record 
their stop dates or confirm if they are still continuing. In addition, any AEs or research-
related injuries (RRIs) occurring up to 48 hours after the blood draw will be recorded.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

For subjects at designated sites who have consented to the additional blood draw, collect 
a whole blood sample of approximately 20 to 50 mL for exploratory testing (see details 
below*).

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
*For sites to participate in collection of blood samples for exploratory purposes, the 
following additional procedures will be completed:
Obtain written informed consent from subjects agreeing to the additional blood draw.
Obtain additional blood sample of 20 to 50 mL for exploratory purposes. Maximum
amount of blood to be obtained at this visit for those subjects participating is up to 
70 mL.
6.3.5. Visit 13: Month 12 After Vaccination 4 (±30 Days)

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Record any AEs or RRIs that occurred up to 48 hours after the previous blood draw as 
described in Section 8 and the Schedule of Activities. Review any AEs that were 
ongoing from the previous visit and record their stop dates or confirm if they are still 
continuing.

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
Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.3.6. Visit 14: Month 18 After Vaccination 4 (±30 Days)

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Record any AEs or RRIs that occurred up to 48 hours after the previous blood draw as 
described in Section 8 and the Schedule of Activities. Review any AEs that were 
ongoing from the previous visit and record their stop dates or confirm if they are still 
continuing.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.3.7. Visit 15: Month 24 After Vaccination 4 (±30 Days)

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Record any AEs or RRIs that occurred up to 48 hours after the previous blood draw as 
described in Section 8 and the Schedule of Activities. Review any AEs that were 
ongoing from the previous visit and record their stop dates or confirm if they are still 
continuing.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.3.8. Visit 16: Month 30 After Vaccination 4 (±30 Days)

Ensure that the subject continues to be eligible for the study (see Section 6.5).

Record any AEs or RRIs that occurred up to 48 hours after the previous blood draw as 
described in Section 8 and the Schedule of Activities. Review any AEs that were 

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ongoing from the previous visit and record their stop dates or confirm if they are still 
continuing.

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Schedule an appointment for the subject to return for the next study visit.

Complete the source documents.

Complete the CRF.
6.3.9. Visit 17: Month 36 After Vaccination 4 (±30 Days)

Record any AEs or RRIs that occurred up to 48 hours after the previous blood draw as 
described in Section 8 and the Schedule of Activities. Review any AEs that were 
ongoing from the previous visit and record their stop dates or confirm if they are still 
continuing. 

Collect a blood sample of approximately 20 mL for immunogenicity testing (Section 7.2).

Complete the source documents.

Complete the CRF.
6.4. Severe Reaction Assessment and Unscheduled Visits
If a severe local reaction (Section 7.1.1.2), severe systemic event (Section 7.1.1.3), or severe 
fever (Section 7.1.1.4) is reported between scheduled visits, a telephone contact must occur 
as soon as possible between the investigator or medically qualified member of the study staff 
and the subject to assess if an unscheduled site visit is required. A site visit must be 
scheduled as soon as possible to assess the extent of the reaction, unless:

The subject is unable to attend the unscheduled visit, or

The reaction is no longer present at the time of the telephone contact, or

The investigator confirms that the data were entered into the e-diary in error by the 
subject.
This telephone contact will be recorded in the subject source documents and the CRF.
If the subject is unable to attend the unscheduled visit, reactions must be assessed at the next 
scheduled visit.
Reactions should be assessed by the investigator or medically qualified member of the study 
staff. For the purpose of reaction assessment, a medically qualified member of the study staff 
is a study physician or a study nurse, as applicable to the investigator’s local practice. If a 

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severe local reaction, severe systemic event, or severe fever is suspected at a scheduled visit, 
the same assessments should be conducted.
Assessment should include the following:

Measure oral temperature (C or F).

Measure minimum and maximum diameter (cm) of any redness present.

Measure minimum and maximum diameter (cm) of any swelling present.

Assess any pain present in accordance with the grades provided in Section 7.1.1.3.

Assess for lymphadenopathy associated with any local reaction present.

Assess any systemic event (vomiting, diarrhea, headache, fatigue, new or worsening 
muscle pain, and new or worsening joint pain) that is present.

Determine whether any of the reported symptoms or signs meet Grade 4 and/or SAE 
reporting criteria (refer to Section 7.1.1.2, Section 7.1.1.3, Section 7.1.1.4, and 
Section 8.6).
An extra visit may also be scheduled for assessment of any other AE.
6.5. Subject Withdrawal
Subject eligibility must be confirmed at each visit in order to continue in the study. Inclusion 
criteria 2, 3 ,4, and 5 and exclusion criteria 4, 7, 8, 9, 11, 12, 13, 15, and 16 should be 
considered before a subject is withdrawn at any point during the study. Exclusion criteria 3, 
5, 6, and 14 should be considered before a subject is withdrawn during the vaccination phase 
(refer to Section 4).
During the extension stage, inclusion criteria 1, 6, and 7 for the extension stage and exclusion 
criteria 1, 4, 15, and 19 for the extension stage should be considered before a subject is 
withdrawn at any point during the study.  Subjects may withdraw from the study at any time 
at their own request, or they may be withdrawn at any time at the discretion of the 
investigator or sponsor for safety or behavioral reasons, or the inability of the subject to 
comply with the protocol-required schedule of study visits or procedures at a given study 
site. If a subject experiences a confirmed Grade 3 or 4 local reaction, a confirmed Grade 4
systemic event, or a confirmed Grade 4 fever or SAE determined to be related to the 
administration of the investigational product, further vaccinations will be discontinued in that 
subject.
If a subject does not return for a scheduled visit, every effort should be made to contact the 
subject. Procedures for managing lost-to-follow-up subjects are described in the SRM. In any 
circumstance, every effort should be made to document subject outcome, if possible. The 
investigator should inquire about the reason for withdrawal, request the subject to return all 

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study-related materials (ie, e-diary), request the subject to return for a final visit, if applicable 
(as defined below), and follow up with the subject regarding any unresolved AEs.
When a subject discontinues or meets criteria for withdrawal, the investigator will notify the 
sponsor and, when possible, will perform the procedures for the next scheduled visit.
Additionally, where possible, the investigator must contact the subject by phone 12 months 
after his or her third vaccination and invite the subject to visit the study site to conduct the 
procedures detailed for Visit 9 as per Section 6.1.9 (excluding subjects entering the extension
stage). If the subject is unwilling or unable to come in for the visit, the investigator will 
record whether any newly diagnosed chronic medical conditions or SAEs have occurred 
since the last study visit.
If the subject withdraws from the study, and also withdraws consent for disclosure of future 
information, no further evaluations should be performed, and no additional data should be 
collected. The sponsor may retain and continue to use any data collected before such 
withdrawal of consent.
Additional subjects will not be enrolled to replace discontinued/withdrawn subjects following 
receipt of the first study vaccination.
7. ASSESSMENTS
Every effort should be made to ensure that the protocol-required tests and procedures are 
completed as described. However, it is anticipated that from time to time there may be 
circumstances, outside of the control of the investigator, that may make it unfeasible to 
perform the test. In these cases the investigator will take all steps necessary to ensure the 
safety and well-being of the subject. When a protocol-required test cannot be performed, the 
investigator will document the reason for this and any corrective and preventive actions that 
he or she has taken to ensure that normal processes are adhered to as soon as possible. The 
study team will be informed of these incidents in a timely fashion.
7.1. Safety
Safety parameters will be assessed as described in the Schedule of Activities, Section 6, 
Section 8, and below.
A medical history and physical examination including measurement of vital signs will be 
performed on all subjects at Visit 1 to establish a baseline. At all subsequent visits during the 
original portion of the study, and at Visits 9, 10, and 11 during the extension stage, a targeted 
physical examination (including vital sign measurement if appropriate) will be performed, to 
evaluate any clinically significant abnormalities based on history and the participant’s 
self-reported symptoms or complaints since the previous visit. Significant medical history 
and observations from the physical examination will be documented in the CRF.
The safety parameters include e-diary reports of local reactions and systemic events that 
occur in the 14 days (or 7 days for the first vaccination for subjects on the Day 1, 8, and 30 

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regimen) after investigational product administration. These prospectively collected 
occurrences of local reactions and systemic events are graded as described in Section 7.1.1.
Acute reactions within the first 30 minutes after investigational product administration will 
be assessed and documented in the AE CRF.
In addition, AEs, newly diagnosed chronic medical conditions, SAEs, and RRIs are 
collected, recorded, and reported as defined in Section 8.
7.1.1. Electronic Diary
The subject will be issued an e-diary, based on a smartphone or equivalent technology, and 
will be asked to monitor and record local reactions, systemic events, and the use of 
antipyretics/pain medication to treat symptoms for 14 days following vaccination (or 7 days 
following the first vaccination for subjects on the Day 1, 8, and 30 regimen). The e-diary 
allows recording of these assessments only within a fixed time window, thus providing the 
accurate representation of the subject’s experience at that time. Data on local reactions, 
systemic events, and use of antipyretics/pain medication to treat symptoms reported on the 
e-diary will be transferred electronically to the e-diary vendor, where they will be available 
for review by investigators and the Pfizer clinicians at all times via an Internet-based portal. 
E-diary data review is ongoing during subject e-diary data-entry periods (7 days after the first 
vaccination and 14 days after the second, third, and fourth [if applicable] vaccinations) via an 
Internet-based portal.
At intervals agreed to by the vendor and Pfizer, these data will be transferred electronically 
into Pfizer's database for analysis and reporting. These data do not need to be reported by the 
investigator in the CRF.
Investigators (or designee) will be required to review the e-diary data online at frequent 
intervals as part of the ongoing safety review.
The investigator or designee must contact the subject in order to obtain stop dates for any 
ongoing local reactions or systemic events or continuing use of antipyretic/pain medication to 
treat symptoms on the last day that the e-diary was completed. The stop dates should be 
documented in the source documents and the information entered in the CRF.
7.1.1.1. Grading Scales
The grading scales used in this study to assess local reactions and systemic events as 
described below are derived from the Food and Drug Administration (FDA) Center for 
Biologics Evaluation and Research (CBER) guidelines on toxicity grading scales for healthy 
adult volunteers enrolled in preventive vaccine clinical trials.37
7.1.1.2. Local Reactions
During the e-diary reporting period, subjects will be asked to assess redness, swelling, and 
pain at the injection site and to record the symptoms in the e-diary. If a local reaction 
persists beyond the end of the e-diary period following vaccination, the subject will be 

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requested to report that information and any new AEs that develop to the investigator. The 
investigator will enter this additional information in the CRF.
Redness and swelling will be measured and recorded in measuring device units (range: 1 to 
21+), and then categorized during analysis as absent, mild, moderate, or severe based on the 
grading scale in Table 2. Measuring device units can be converted to centimeters according 
to the following formula: 1 measuring device unit = 0.5 cm. Pain at the vaccine injection site 
will be assessed by the subject as absent, mild, moderate, or severe according the grading 
scale in Table 2.
A subject with a severe local reaction at the injection site will be prompted to contact the 
investigator to assess the reaction and perform an unscheduled visit as appropriate. Only an 
investigator or medically qualified person is able to classify a subject’s local reaction as 
Grade 4. If a subject experiences a suspected Grade 4 local reaction, the investigator must 
immediately notify the sponsor. If a subject experiences a confirmed Grade 3 or 4 local 
reaction determined to be related to the administration of the investigational product, further 
vaccinations will be discontinued in that subject.
Table 2.
Local Reaction Grading Scale
Mild 
(Grade 1)
Moderate 
(Grade 2)
Severe 
(Grade 3)
Grade 4
Pain
Does not interfere 
with activity
Interferes with activity Prevents daily activity Emergency room visit 
or hospitalization
Erythema/
Redness
2.5 cm to 5.0 cm (5 to 
10 measuring device 
units)
>5.0 cm to 10.0 cm 
(11 to 20 measuring 
device units)
>10 cm 
(21 measuring 
device units)
Necrosis or exfoliative 
dermatitis
Induration/
Swelling
2.5 cm to 5.0 cm (5 to 
10 measuring device 
units)
>5.0 cm to 10.0 cm 
(11 to 20 measuring 
device units)
>10 cm 
(21 measuring 
device units)
Necrosis
7.1.1.3. Systemic Events
During the e-diary reporting period, subjects will be asked to assess vomiting, diarrhea, 
headache, fatigue, new or worsening muscle pain, and new or worsening joint pain and to 
record the symptoms in the e-diary. The symptoms will be assessed by the subject as absent, 
mild, moderate, or severe according to the grading scale in Table 3. Only an investigator or 
medically qualified person is able to classify a subject’s systemic events as Grade 4. If a 
subject experiences a suspected Grade 4 systemic event, the investigator must immediately 
notify the sponsor. If a subject experiences a confirmed Grade 4 systemic event determined 
to be related to the administration of the investigational product, further vaccinations will be 
discontinued in that subject. Further, if a systemic event persists beyond the end of the 
e-diary period following vaccination, the subject will be requested to report that information 
and any new AEs that develop to the investigator. The investigator will enter this additional 
information in the CRF.

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Table 3.
Systemic Event Grading Scale
Mild 
(Grade 1)
Moderate 
(Grade 2)
Severe 
(Grade 3)
Grade 4
Vomiting
1-2 times in 
24 hours
>2 times in 24 hours Requires IV 
hydration
Emergency room visit or 
hospitalization for 
hypotensive shock
Diarrhea
2-3 loose stools in 
24 hours
4-5 loose stools in 
24 hours
6 loose stools in 
24 hours
Emergency room visit or 
hospitalization
Headache
No interference 
with activity
Some interference 
with activity
Significant; prevents 
daily activity
Emergency room visit or 
hospitalization
Fatigue/ 
Tiredness
No interference 
with activity
Some interference 
with activity
Significant; prevents 
daily activity
Emergency room visit or 
hospitalization
New or 
worsening 
muscle pain
No interference 
with activity
Some interference 
with activity
Significant; prevents 
daily activity
Emergency room visit or 
hospitalization
New or 
worsening 
joint pain
No interference 
with activity
Some interference 
with activity
Significant; prevents 
daily activity
Emergency room visit or 
hospitalization
Abbreviation: IV = intravenous.
7.1.1.4. Fever
In order to record information on fever, a digital thermometer will be given to the subject 
with instructions on how to measure oral temperature at home. Temperature will be 
collected in the e-diary in the evening daily during the e-diary reporting period. It will also 
be collected at any time during the e-diary data collection periods that fever is suspected. If a 
fever persists beyond the end of the e-diary period following vaccination, the subject will be 
requested to report that information and any new AEs that develop to the investigator. The 
investigator will enter this additional information in the CRF. Fever is defined as an oral 
temperature of 38.0C (100.4F). The highest temperature for each day will be recorded in 
the e-diary. In the event of a fever, temperature will be collected daily until fever has 
resolved (1 day of temperature less than 38.0C [100.4F]). Temperature will be measured 
and recorded to 1 decimal place and then categorized during analysis according to the scale 
shown in Table 4.
Table 4.
Scale for Fever
Mild (Grade 1)
38.0-38.4C (100.4-101.1F)
Moderate (Grade 2)
38.5-38.9C (101.2-102.0F)
Severe (Grade 3)
39.0-40.0C (102.1-104.0F)
Grade 4
>40.0C (>104.0F)
If a subject experiences a confirmed Grade 4 fever determined to be related to the 
administration of the investigational product, further vaccinations will be discontinued in that 
subject.

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7.1.2. Triggered Safety Data Review Rule
In light of the occurrence of cases of Grade 3 injection site erythema in subjects receiving a 
different formulation of the C difficile vaccine on a Day 1, 8, and 30 regimen (Section 1.2.3), 
a triggered safety data review rule will apply during the course of the study for e-diary–
prompted injection site erythema. This is designed to trigger if the estimated lower limit of 
the 1-sided 97.5% CI around the rate of Grade 3 injection site erythema is greater than 2.1% 
among recipients of active vaccine.
The rule will be assessed in a blinded manner and will apply separately to each dose of 
vaccine (ie, Doses 1, 2, and 3) and each vaccination regimen (ie, Day 1, 8, and 30 or 
Month 0, 1, and 6), irrespective of vaccine allocation. The rule will not be applied to the 
fourth dose during the extension stage. E-diary data confirmed to be entered by the subject in 
error will not contribute towards the rule.
The triggered safety data review rule will have been met if 1 confirmed case of Grade 4
injection site erythema is reported. If more than 1 confirmed case of Grade 3 injection site 
erythema is reported, the rule will be assessed as follows. The number of confirmed cases 
(occurring after the specified dose number in the specified vaccination regimen) will be 
compared with the total number of subjects who have received the specified dose number in 
the specified vaccination regimen (in the knowledge that approximately 1 of 7 subjects will 
have received placebo). If the total number of subjects is less than or equal to the 
corresponding value shown in Table 5, the triggered safety data review rule will have been 
met.
Table 5.
Triggered Safety Data Review Rule Criteria for Grade 3 Injection Site 
Erythema After the Specified Dose Number in the Specified Vaccination 
Regimen
Number of Confirmed Case(s) of 
Grade 3 Injection Site Erythema
Number of Subjects Having Received the Specified Dose Number in 
the Specified Vaccination Regimen
1
Rule does not apply
2
14
3
35
4
63
5
94
6
126
7
161
8
199
9
236
10
276
11
315
12
355
13
397
14
427
If the rule is met, the Pfizer IRC will conduct an expedited review of the available safety data 
in an unblinded manner to determine whether enrollment of new subjects to the impacted 

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vaccination regimen and administration of study injections to subjects already enrolled in the 
impacted vaccination regimen should be halted temporarily or whether any other measures 
should be taken to ensure subject safety. If the IRC determines that enrollment and 
vaccination in the impacted regimen should be halted temporarily, the data will also be 
reviewed by the E-DMC. Enrollment and vaccination of subjects in the impacted vaccination 
regimen will not recommence until the E-DMC evaluates all available safety data and 
provides recommendations and the sponsor notifies sites that these activities may continue 
(refer to Section 9.5). In the meantime, all other routine study conduct activities such as 
ongoing data entry, reporting of AEs, subject e-diary completion, and subject follow-up must 
continue. If only 1 vaccination regimen is impacted by the triggered safety data review rule, 
enrollment and vaccination in the other vaccination regimen will continue per protocol.
7.2. Immunogenicity
Serum samples will be obtained for immunogenicity testing as follows:

For subjects assigned to the Day 1, 8, and 30 regimen, on Day 1, Day 8, Day 15, Day 30, 
Day 37, Month 2, Month 4, Month 7, and Month 13.

For subjects assigned to the Month 0, 1, and 6 regimen, on Day 1, Day 30, Day 37, 
Month 2, Month 6, Day 187, Month 7, Month 12, and Month 18.

During the extension stage: at the fourth dose (immediately before the fourth dose), on
Days 8 and 30 after the fourth dose, and at Months 6, 12, 18, 24, 30, and 36 after the 
fourth dose.
Both toxin A– and toxin B–specific neutralizing antibody levels will be measured. A 
minimum of 10 mL and up to 20 mL of blood will be collected (prior to vaccination if at a 
vaccination visit) for each measurement to allow for adequate volume required for repeat 
testing or additional antigen-specific immunogenicity testing to be performed.
An additional blood sample of approximately 40 mL will be obtained at Visit 6 (Month 2) for 
subjects on the Day 1, 8, and 30 regimen and Visit 4 (Month 2) for subjects on the Month 0, 
1, and 6 regimen for use in development, qualification, and validation of C difficile serologic 
assays and reagents.
An additional optional whole blood sample of approximately 20 to 50 mL will be obtained 
from subjects at designated sites for exploratory purposes as follows:

Designated sites participating in the month regimen will be asked to obtain additional 
blood samples from subjects who have signed an additional ICD for this procedure at 
Visit 12 for exploratory purposes. 
Refer to the SRM for details regarding serum sample management and storage.

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7.3.
 
 
 
 
 
 
8. ADVERSE EVENT REPORTING
8.1. Adverse Events
All observed or volunteered AEs regardless of vaccine group or suspected causal relationship 
to the investigational product(s) will be reported as described in the following sections.
For all AEs, the investigator must pursue and obtain information adequate both to determine 
the outcome of the AE and to assess whether it meets the criteria for classification as an SAE
requiring immediate notification to Pfizer or its designated representative. For all AEs, 
sufficient information should be obtained by the investigator to determine the causality of the 
AE. The investigator is required to assess causality. Follow-up by the investigator may be 
required until the event or its sequelae resolve or stabilize at a level acceptable to the 
investigator, and Pfizer concurs with that assessment.
As part of ongoing safety reviews conducted by the sponsor, any nonserious AE that is 
determined by the sponsor to be serious will be reported by the sponsor as an SAE. To assist 
in the determination of case seriousness, further information may be requested from the 
investigator to provide clarity and understanding of the event in the context of the clinical 
study.
8.2. Reporting Period
For SAEs, the active reporting period to Pfizer or its designated representative begins from 
the time that the subject provides informed consent, which is obtained prior to the subject’s 
participation in the study, ie, prior to undergoing any study-related procedure and/or 
receiving investigational product, through and including Visit 8 (Month 7 for subjects on the 
Day 1, 8, and 30 regimen and Month 12 for subjects on the Month 0, 1, and 6 regimen).
During the extension stage, the active reporting period for SAEs to Pfizer or its designated 
representative begins from the time that the subject provides informed consent for the 
extension stage, which is obtained prior to the subject’s participation in the extension stage, 
ie, prior to undergoing any study-related procedure and/or receiving investigational product, 
through and including Visit 12 (6 months after the fourth dose).
CCI
CCI

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SAEs occurring to a subject after the active reporting period has ended should be reported to 
the sponsor if the investigator becomes aware of them; at a minimum, all SAEs that the 
investigator believes have at least a reasonable possibility of being related to investigational 
product are to be reported to the sponsor.
All AEs should be recorded on the CRF from the time the subject provides informed consent 
until 1 month after the third vaccination (Visit 6 [Month 2] for subjects on the Day 1, 8, and 
30 regimen and Visit 7 [Month 7] for subjects on the Month 0, 1, and 6 regimen).
During the extension stage, all AEs should be recorded on the CRF from the time the subject 
provides informed consent until 1 month after the fourth vaccination (Visit 11).
Any AE occurring up to 48 hours after each subsequent blood draw (Visit 6 [Month 2], 
Visit 7 [Month 4], Visit 8 [Month 7], and Visit 9 [Month 13] for subjects on the Day 1, 8, and 
30 regimen and Visit 7 [Month 7], Visit 8 [Month 12], and Visit 9 [Month 18] for subjects on 
the Month 0, 1, and 6 regimen) must be recorded on the CRF.
During the latter part of the extension stage, any AE (including RRIs) occurring up to 48 
hours after the blood draw at Visits 12, 13, 14, 15, 16, and 17 (Months 6, 12, 18, 24, 30, and 
36 after the fourth dose) must be recorded on the CRF.
The investigator is required to assess whether the AE may be related to the subject’s 
participation in the study.
 
 
8.3. Definition of an Adverse Event
An AE is any untoward medical occurrence in a clinical investigation subject administered a 
product or medical device; the event need not necessarily have a causal relationship with the 
treatment or usage. Examples of AEs include but are not limited to: 

Abnormal test findings;

Clinically significant symptoms and signs;

Changes in physical examination findings;

Hypersensitivity;

Progression/worsening of underlying disease;

Drug abuse;

Drug dependency.
CCI

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Additionally, they may include the signs or symptoms resulting from:

Drug overdose;

Drug withdrawal;

Drug misuse; 

Drug interactions;

Extravasation;

Exposure during pregnancy (EDP);

Exposure via breastfeeding;

Medication error;

Occupational exposure.
8.4. Medication Errors
Medication errors may result, in this study, from the administration of the wrong 
investigational product, to the wrong subject, at the wrong time, or at the wrong dosage 
strength. Such medication errors occurring to a study participant are to be captured on the 
medication error CRF, which is a specific version of the AE page, and on the SAE form 
when appropriate. In the event of medication dosing error, the sponsor should be notified 
immediately.
Medication errors are reportable irrespective of the presence of an associated AE/SAE, 
including:

Medication errors involving subject exposure to the investigational product;

Potential medication errors or uses outside of what is foreseen in the protocol that do 
or do not involve the participating subject.
Whether or not the medication error is accompanied by an AE, as determined by the 
investigator, the medication error is captured on the medication error version of the AE page 
and, if applicable, any associated AE(s) are captured on an AE CRF page.
Other examples include, but are not limited to:

The administration of expired investigational product;

The administration of an incorrect investigational product;

The administration of an incorrect dosage;

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
The administration of investigational product that has undergone temperature 
excursion from the specified storage range, unless it is determined by the sponsor that 
the investigational product under question is acceptable for use.
8.5. Abnormal Test Findings
The criteria for determining whether an abnormal objective test finding should be reported as 
an AE are as follows: 

Test result is associated with accompanying symptoms; and/or

Test result requires additional diagnostic testing or medical/surgical intervention;
and/or

Test result leads to a change in study dosing or discontinuation from the study, 
significant additional concomitant drug treatment, or other therapy; and/or

Test result is considered to be an AE by the investigator or sponsor.
Merely repeating an abnormal test, in the absence of any of the above conditions, does not 
constitute an AE. Any abnormal test result that is determined to be an error does not require 
reporting as an AE.
8.6. Serious Adverse Events
An SAE is any untoward medical occurrence at any dose that:

Results in death;

Is life-threatening (immediate risk of death);

Requires inpatient hospitalization or prolongation of existing hospitalization;

Results in persistent or significant disability/incapacity (substantial disruption of the 
ability to conduct normal life functions);

Results in congenital anomaly/birth defect.

Lack of efficacy in an approved indication should be reported as an SAE.
Medical and scientific judgment is exercised in determining whether an event is an important 
medical event. An important medical event may not be immediately life-threatening and/or 
result in death or hospitalization. However, if it is determined that the event may jeopardize 
the subject or may require intervention to prevent one of the other AE outcomes, the 
important medical event should be reported as serious.

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Examples of such events are intensive treatment in an emergency room or at home for 
allergic bronchospasm; blood dyscrasias or convulsions that do not result in hospitalization; 
or development of drug dependency or drug abuse.
8.6.1. Protocol-Specified Serious Adverse Events 
There are no protocol-specified SAEs in this study.  All SAEs will be reported by the 
investigator as described in previous sections, and will be handled as SAEs in the safety 
database (see the section on Serious Adverse Event Reporting Requirements).
8.6.2. Potential Cases of Drug-Induced Liver Injury
Liver function tests (LFTs) are not required as a routine safety monitoring procedure in this 
study. However, should an investigator deem it necessary to run LFTs because of clinical 
sign/symptom presentation in a subject, such LFT results should be handled and followed up 
as described below.
Abnormal values in aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) 
levels concurrent with abnormal elevations in total bilirubin level that meet the criteria 
outlined below in the absence of other causes of liver injury are considered potential cases of 
drug-induced liver injury (potential Hy’s law cases) and should always be considered 
important medical events.
The threshold of laboratory abnormalities for a potential case of drug-induced liver injury 
depends on the subject’s individual baseline values and underlying conditions. Subjects who 
present with the following laboratory abnormalities should be evaluated further to 
definitively determine the etiology of the abnormal laboratory values:

Subjects with AST or ALT and total bilirubin baseline values within the normal range 
who subsequently present with AST or ALT values 3 times the upper limit of 
normal (× ULN) concurrent with a total bilirubin value 2 × ULN with no evidence 
of hemolysis and an alkaline phosphatase value 2 × ULN or not available;

For subjects with preexisting ALT OR AST OR total bilirubin values above the 
ULN, the following threshold values should be used in the definition mentioned 
above:

For subjects with preexisting AST or ALT baseline values above the normal
range: AST or ALT values 2 times the baseline values and 3 × ULN, or 
8 × ULN (whichever is smaller).
Concurrent with

For subjects with preexisting values of total bilirubin above the normal range: 
Total bilirubin level increased from baseline by an amount of at least 1 × ULN or
if the value reaches ≥3 × ULN (whichever is smaller).

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The subject should return to the investigational site and be evaluated as soon as possible, 
preferably within 48 hours from awareness of the abnormal results. This evaluation should 
include laboratory tests, detailed history, and physical assessment.
In addition to repeating measurements of AST and ALT, laboratory tests should include 
albumin, creatine kinase, total bilirubin, direct and indirect bilirubin, gamma-glutamyl 
transferase, prothrombin time (PT)/international normalized ratio (INR), and alkaline 
phosphatase. A detailed history, including relevant information, such as review of ethanol, 
acetaminophen, recreational drug and supplement consumption, family history, occupational 
exposure, sexual history, travel history, history of contact with a jaundiced person, surgery, 
blood transfusion, history of liver or allergic disease, and work exposure, should be collected.
Further testing for acute hepatitis A, B, or C infection and liver imaging (eg, biliary tract) 
may be warranted. All cases confirmed on repeat testing as meeting the laboratory criteria 
defined above, with no other cause for LFT abnormalities identified at the time, should be 
considered potential Hy’s law cases irrespective of availability of all the results of the 
investigations performed to determine etiology of the abnormal LFTs. Such potential Hy’s 
law cases should be reported as SAEs.
8.7. Research-Related Injury
During the latter part of the extension stage, for up to 48 hours after the blood draw at 
Visits 12, 13, 14, 15, 16, and 17 (Months 6, 12, 18, 24, 30, and 36 after the fourth dose),
should a subject, in the investigator’s opinion, suffer a medically important RRI caused by 
his or her participation in the protocol, Pfizer or its representative must be informed 
immediately.
A medically important RRI is any untoward medical occurrence that:

Results in death;

Is life-threatening (immediate risk of death);

Requires inpatient hospitalization or prolongation of existing hospitalization;

Results in persistent or significant disability/incapacity;

Results in congenital anomaly/birth defect.
Medical and scientific judgment is exercised in determining whether an injury is an 
important medical event.  An important medical event may not be immediately 
life-threatening and/or result in death or hospitalization.  However, if it is determined that the 
event may jeopardize the subject or may require intervention to prevent one of the other 
outcomes listed in the definition above, the important medical event should be reported as an
RRI.
An investigator may be requested by Pfizer to obtain specific additional follow-up 
information in an expedited fashion.  In general, this will include a description of the injury 

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in sufficient detail to allow for a complete medical assessment of the case and independent 
determination of possible causality.  Information on other possible causes of the event, such 
as concomitant medications, vaccines, and/or illnesses, must be provided.  In the case of a 
subject death, a summary of available autopsy findings must be submitted as soon as possible 
to Pfizer or its designated representative.
8.8. Hospitalization
Hospitalization is defined as any initial admission (even less than 24 hours) in a hospital or 
equivalent healthcare facility or any prolongation of an existing admission. Admission also 
includes transfer within the hospital to an acute/intensive care unit (eg, from the psychiatric 
wing to a medical floor, medical floor to a coronary care unit, or neurological floor to a 
tuberculosis unit). An emergency room visit does not necessarily constitute a hospitalization; 
however, the event leading to the emergency room visit should be assessed for medical 
importance.
Hospitalization does not include the following:

Rehabilitation facilities;

Hospice facilities;

Respite care (eg, caregiver relief);

Skilled nursing facilities;

Nursing homes;

Same-day surgeries (as outpatient/same-day/ambulatory procedures).
Hospitalization or prolongation of hospitalization in the absence of a precipitating, clinical 
AE is not in itself an SAE. Examples include: 

Admission for treatment of a preexisting condition not associated with the 
development of a new AE or with a worsening of the preexisting condition (eg, for 
workup of persistent pretreatment laboratory abnormality);

Social admission (eg, subject has no place to sleep);

Administrative admission (eg, for yearly physical examination);

Protocol-specified admission during a study (eg, for a procedure required by the study 
protocol);

Optional admission not associated with a precipitating clinical AE (eg, for elective 
cosmetic surgery);

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
Hospitalization for observation without a medical AE;

Preplanned treatments or surgical procedures. These should be noted in the baseline 
documentation for the entire protocol and/or for the individual subject.
Diagnostic and therapeutic noninvasive and invasive procedures, such as surgery, should not 
be reported as AEs. However, the medical condition for which the procedure was performed 
should be reported if it meets the definition of an AE. For example, an acute appendicitis 
that begins during the AE reporting period should be reported as the AE, and the resulting 
appendectomy should be recorded as treatment of the AE.
8.9. Severity Assessment
GRADE 
If required on the AE CRFs, the investigator will use the adjectives MILD, 
MODERATE, SEVERE, or LIFE-THREATENING to describe the maximum 
intensity of the AE. For purposes of consistency, these intensity grades are 
defined as follows:
1
MILD
Does not interfere with subject's usual function.
2
MODERATE
Interferes to some extent with subject's usual function.
3
SEVERE
Interferes significantly with subject's usual function.
4
LIFE-
THREATENING
Life-threatening consequences; urgent intervention 
indicated.
Note the distinction between the severity and the seriousness of an AE.  A severe event is not 
necessarily an SAE.  For example, a headache may be severe (interferes significantly with 
the subject's usual function) but would not be classified as serious unless it met one of the 
criteria for SAEs, listed above.
8.10. Causality Assessment
The investigator’s assessment of causality must be provided for all AEs (serious and 
nonserious); the investigator must record the causal relationship in the CRF, as appropriate, 
and report such an assessment in accordance with the SAE reporting requirements if 
applicable. An investigator’s causality assessment is the determination of whether there 
exists a reasonable possibility that the investigational product caused or contributed to an 
AE; generally the facts (evidence) or arguments to suggest a causal relationship should be 
provided. If the investigator does not know whether or not the investigational product caused 
the event, then the event will be handled as “related to investigational product” for reporting 
purposes, as defined by the sponsor (see the section on Reporting Requirements). If the 
investigator's causality assessment is "unknown but not related to investigational product," 
this should be clearly documented on study records.
In addition, if the investigator determines that an SAE is associated with study procedures, 
the investigator must record this causal relationship in the source documents and CRF, as 

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appropriate, and report such an assessment in accordance with the SAE reporting 
requirements, if applicable.
8.11. Exposure During Pregnancy
For both unapproved/unlicensed products and for marketed products, an EDP occurs if:
1. A female becomes, or is found to be, pregnant either while receiving or having been 
exposed (eg, because of treatment or environmental exposure) to the investigational 
product; or the female becomes or is found to be pregnant after discontinuing and/or 
being exposed to the investigational product;
An example of environmental exposure would be a case involving direct contact with 
a Pfizer product in a pregnant woman (eg, a nurse reports that she is pregnant and has 
been exposed to chemotherapeutic products).
2. A male has been exposed (eg, because of treatment or environmental exposure) to the 
investigational product prior to or around the time of conception and/or is exposed 
during his partner’s pregnancy.
If a study subject or study subject’s partner becomes or is found to be pregnant during the 
study subject’s treatment with the investigational product, the investigator must submit this 
information to the Pfizer drug safety unit on an SAE report form and an EDP supplemental 
form, regardless of whether an SAE has occurred. In addition, the investigator must submit 
information regarding environmental exposure to a Pfizer product in a pregnant woman (eg, a 
subject reports that she is pregnant and has been exposed to a cytotoxic product by inhalation 
or spillage) using the EDP supplemental form. This must be done irrespective of whether an 
AE has occurred and within 24 hours of awareness of the exposure. The information 
submitted should include the anticipated date of delivery (see below for information related 
to termination of pregnancy).
Follow-up is conducted to obtain general information on the pregnancy and its outcome for 
all EDP reports with an unknown outcome. The investigator will follow the pregnancy until 
completion (or until pregnancy termination) and notify Pfizer of the outcome as a follow-up 
to the initial EDP supplemental form. In the case of a live birth, the structural integrity of the 
neonate can be assessed at the time of birth. In the event of a termination, the reason(s) for 
termination should be specified and, if clinically possible, the structural integrity of the 
terminated fetus should be assessed by gross visual inspection (unless preprocedure test 
findings are conclusive for a congenital anomaly and the findings are reported).
If the outcome of the pregnancy meets the criteria for an SAE (ie, ectopic pregnancy, 
spontaneous abortion, intrauterine fetal demise, neonatal death, or congenital anomaly [in a 
live-born baby, a terminated fetus, an intrauterine fetal demise, or a neonatal death]), the 
investigator should follow the procedures for reporting SAEs.
Additional information about pregnancy outcomes that are reported as SAEs follows: 

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
Spontaneous abortion includes miscarriage and missed abortion;

Neonatal deaths that occur within 1 month of birth should be reported, without regard 
to causality, as SAEs. In addition, infant deaths after 1 month should be reported as 
SAEs when the investigator assesses the infant death as related or possibly related to 
exposure to the investigational product.
Additional information regarding the EDP may be requested by the investigator. Further 
follow-up of birth outcomes will be handled on a case-by-case basis (eg, follow-up on 
preterm infants to identify developmental delays). In the case of paternal exposure, the 
investigator will provide the study subject with the Pregnant Partner Release of Information 
Form to deliver to his partner. The investigator must document in the source documents that 
the subject was given the Pregnant Partner Release of Information Form to provide to his 
partner.
8.12. Occupational Exposure
An occupational exposure occurs when during the performance of job duties, a person 
(whether a healthcare professional or otherwise) gets in unplanned direct contact with the 
product, which may or may not lead to the occurrence of an AE.
An occupational exposure is reported to the drug safety unit within 24 hours of the 
investigator’s awareness, using the SAE report form, regardless of whether there is an 
associated AE/SAE. Since the information does not pertain to a subject enrolled in the study, 
the information is not reported on a CRF; however, a copy of the completed SAE report form 
is maintained in the investigator site file.
8.13. Withdrawal Due to Adverse Events (See Also the Section on Subject Withdrawal)
Withdrawal due to AEs should be distinguished from withdrawal due to other causes, 
according to the definition of AE noted earlier, and recorded on the appropriate AE CRF 
page.
When a subject withdraws because of an SAE, the SAE must be reported in accordance with 
the reporting requirements defined below.
8.14. Eliciting Adverse Event Information
The investigator is to report all directly observed AEs and all AEs spontaneously reported by 
the study subject. In addition, each study subject will be questioned about AEs.
8.15. Reporting Requirements
Each AE is to be assessed to determine if it meets the criteria for SAEs. If an SAE occurs, 
expedited reporting will follow local and international regulations, as appropriate.
8.15.1. Serious Adverse Event Reporting Requirements
If an SAE occurs, Pfizer is to be notified within 24 hours of investigator awareness of the 
event.

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In particular, if the SAE is fatal or life-threatening, notification to Pfizer must be made 
immediately, irrespective of the extent of available AE information. This time frame also 
applies to additional new information (follow-up) on previously forwarded SAE reports as 
well as to the initial and follow-up reporting of EDP, exposure via breastfeeding, and 
occupational exposure cases.
In the rare event that the investigator does not become aware of the occurrence of an SAE
immediately (eg, if an outpatient study subject initially seeks treatment elsewhere), the 
investigator is to report the event within 24 hours after learning of it and document the time 
of his or her first awareness of the AE.
For all SAEs, the investigator is obligated to pursue and provide information to Pfizer in 
accordance with the time frames for reporting specified above. In addition, an investigator 
may be requested by Pfizer to obtain specific additional follow-up information in an 
expedited fashion. This information collected for SAEs is more detailed than that captured on 
the AE CRF. In general, this will include a description of the AE in sufficient detail to allow 
for a complete medical assessment of the case and independent determination of possible 
causality. Information on other possible causes of the event, such as concomitant 
medications, vaccines, and/or illnesses, must be provided. In the case of a subject death, a 
summary of available autopsy findings must be submitted as soon as possible to Pfizer or its 
designated representative. 
If the investigator is aware of an SAE following investigational product administration, the 
investigator must contact the Pfizer study physician directly immediately after sending the 
SAE report form to Pfizer as described above. These procedures do not replace any of the 
standard AE reporting requirements as described above. Additional information regarding 
such events and the reporting requirements are included in the SRM.
8.15.2. Nonserious Adverse Event Reporting Requirements
All AEs will be reported on the AE page(s) of the CRF. It should be noted that the form for 
collection of SAE information is not the same as the AE CRF. Where the same data are 
collected, the forms must be completed in a consistent manner. For example, the same AE
term should be used on both forms. AEs should be reported using concise medical 
terminology on the CRFs as well as on the form for collection of SAE information.
8.15.3. Sponsor’s Reporting Requirements to Regulatory Authorities
AE reporting, including suspected unexpected serious adverse reactions, will be carried out 
in accordance with applicable local regulations.
9. DATA ANALYSIS/STATISTICAL METHODS
Detailed methodology for summary and statistical analysis of the data collected in this study 
will be documented in a statistical analysis plan (SAP), which will be finalized before the 
start of any analyses and will be maintained by the sponsor. This document may modify the 
plans outlined in the protocol; however, any major modifications of the primary endpoint 

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definition and/or its analysis will be reflected both in the amended protocol and in the SAP. 
The SAP amendment will follow the protocol amendment.
9.1. Sample Size Determination
The study sample size is not based on statistical hypothesis test considerations. All statistical 
analyses will be descriptive in nature. An estimation approach will be used to assess the 
safety and immunogenicity objectives in the study.
Sample size information for the primary immunogenicity endpoint is included in Table 6. If 
it is assumed that the true proportion of subjects with neutralizing antibody levels greater 
than the specified threshold is 70% and study dropout rate is at most 12%, 183 subjects per 
vaccine group would yield 161 evaluable subjects per vaccine group, giving the study at least 
80% power to demonstrate the proportion is 59.2% or greater (by the lower bound of a 
2-sided 95% confidence limit based on an exact method for 1-sample binomial proportion).
If the proportion of subjects with neutralizing antibody levels greater than the specified 
threshold is assessed in the subset of study participants who are seronegative at baseline
(before Dose 1), estimated to be at least 103 subjects per group, the study will have at least 
80% power to demonstrate that the proportion is 56.0% or greater, assuming a maximum 
discontinuation rate of 12% and a baseline seronegativity rate of 64% (estimated from the 
toxin B–specific neutralizing antibody levels from Study B5091001). Additionally, Table 7
presents the precisions of the 2-sided 95% CI for neutralizing antibody mean concentration 
(in log10) with sample sizes of 103, 161, and 183 with the assumed standard deviations
(SDs).
Table 6.
Minimum Detectable Proportion of Subjects With the Specified Antibody 
Levels (Primary Immunogenicity Endpoint)
Assumed True 
Proportion
Evaluable Subjects Per 
Group
Minimum Detectable Proportion of Subjects With 
Specified Antibody Level(s) With at Least
80% Power
90% Power
0.70
183
59.9%
58.2%
161
59.2%
57.3%
103
56.0%
54.0%
Note: All calculations are based on lower bound of 2-sided 95% confidence limit of the exact method for 
1-sample binomial proportion.

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Table 7.
Precision of the 2-Sided 95% Confidence Interval for Neutralizing Antibody 
Mean Concentration (in Log10)
Assumed True SDa
(in Log10)
Z0.025
(for 95% CI)
Precision of 2-Sided 95% CIb
N = 103
N=161
N=183
0.40
1.96
0.077
0.062
0.058
0.50
1.96
0.097
0.077
0.072
0.60
1.96
0.116
0.093
0.087
0.70
1.96
0.135
0.108
0.101
0.90
1.96
0.174
0.139
0.130
1.10
1.96
0.212
0.170
0.159
1.30
1.96
0.251
0.201
0.188
1.50
1.96
0.290
0.232
0.217
2.00
1.96
0.386
0.309
0.290
a.
Based on immunogenicity results from Study B5091001, the standard deviation (SD) of the neutralization 
titer after log10 transformation from Study B5091001 for the 100-µg C difficile + aluminum hydroxide
vaccine and 200-µg C difficile + aluminum hydroxide vaccine groups ranges from 0.41 to 0.62 for toxin A 
and from 0.78 to 1.40 for toxin B at the Day 37 and Month 7 visits, and from 0.38 to 0.55 for toxin A and 
from 0.35 to 1.77 for toxin B if excluding data from seropositive subjects at baseline.
b.
Precision is the distance from the mean to the confidence limit expressed in log10 of neutralization
units/mL. 
For safety outcomes, Table 8 shows the probability of observing at least 1 AE for a given 
true event rate of a particular AE, at various sample sizes. For example, if a true AE event 
rate is 1.0%, the probability of observing at least 1 such AE is 0.84 in a vaccine group of 
183 subjects.
Table 8.
Probability of Observing at Least 1 AE by Assumed True Event Rates With 
Different Sample Sizes
Assumed True Event 
Rate of an AE
N=183
N=366
N=732
0.50%
0.60
0.84
0.97
0.70%
0.72
0.92
0.99
0.80%
0.77
0.95
>0.99
0.90%
0.81
0.96
>0.99
1.0%
0.84
0.97
>0.99
1.5%
0.94
>0.99
>0.99
2.0%
0.98
>0.99
>0.99
2.5%
0.99
>0.99
>0.99
5.0%
>0.99
>0.99
>0.99

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9.2. Immunogenicity Analysis
The immunogenicity data will be summarized according to the vaccine group as randomized. 
For any C difficile toxin A– or toxin B–specific neutralizing antibody level that is below the 
lower limit of quantitation (LLOQ), the lower limit of detection (LOD), defined as 0.5 × 
LLOQ, will be assigned. No other missing assay data will be imputed in the analyses. All 
immunogenicity analyses will be performed after the imputation of the antibody levels that 
are below the LLOQ.
For the immunogenicity analysis, subjects will be analyzed separately for each planned 
dosing regimen (Days 1, 8, and 30 or Months 0, 1, and 6) for each stage (original planned 
stage [first 3 vaccinations] and extension stage [fourth vaccination]). This applies to all 
analyses outlined in Section 9.2.2 and Section 9.2.3.
9.2.1. Immunogenicity Analysis Population
An evaluable immunogenicity population and a modified intent-to-treat (mITT) population 
for each stage (original planned stage [first 3 vaccinations] and extension stage [fourth 
vaccination]) will be defined for the immunogenicity analyses separately. The evaluable 
immunogenicity populations will be the primary populations for the immunogenicity 
analyses.
In general, the evaluable immunogenicity population will include all subjects who are 
eligible, receive the investigational product to which they are randomized, have blood drawn 
for assay testing within the specified time frame, have valid and determinate assay results for 
the proposed analysis, and have no major protocol violations. The mITT population will 
include all randomized subjects who have at least 1 valid and determinate assay result for the 
proposed analysis.
9.2.2. Analysis of the Primary Immunogenicity Endpoint
The primary immunogenicity endpoints are the proportions of subjects with toxin A–, toxin 
B–, and both toxin A– and toxin B–specific neutralizing antibody levels (neutralization
units/mL) the specified threshold(s) at Day 37 (7 days after Dose 3 for subjects receiving 
the Day 1, 8, and 30 regimen) and at Month 7 (1 month after Dose 3 for subjects receiving 
the Month 0, 1, and 6 regimen). Point estimates and associated 2-sided 95% exact CIs will be 
descriptively summarized for each vaccine group.
9.2.3. Analysis of the Secondary and Other Immunogenicity Endpoints
GMCs of the toxin A– and toxin B–specific neutralizing antibody levels at each blood 
sampling time point will be descriptively summarized for each vaccine group, along with 
associated 2-sided 95% CIs. The CIs will be constructed by back transformation of the 
confidence limits computed for the mean of the logarithmically transformed assay data based 
on Student t distribution.
Point estimates and the associated 2-sided 95% exact CIs will be descriptively summarized
for each vaccine group for the following immunogenicity endpoints: 

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1. Proportions of subjects with toxin A–, toxin B–, and both toxin A– and toxin B–specific 
neutralizing antibody levels (neutralization units/mL) the specified threshold(s) at each 
blood sampling time point except for Day 37 (7 days after Dose 3 for subjects receiving 
the Day 1, 8, and 30 regimen) and Month 7 (1 month after Dose 3 for subjects receiving 
the Month 0, 1, and 6 regimen).
2. Proportions of subjects with 4-fold, 8-fold, 16-fold, and 32-fold rises in either 
toxin A–, toxin B–, or both toxin A– and toxin B–specific neutralizing antibody levels 
compared to baseline at each postbaseline blood sampling time point.
The GMFRs in toxin A– and toxin B–specific neutralizing antibody levels from before 
vaccination to after vaccination will be summarized for each vaccine group by geometric 
means and associated 95% CIs at all postbaseline blood sampling time points. These CIs are 
also computed by back transformation of the CIs using the Student t distribution for mean 
difference of measures on the logarithmically transformed assay results (postvaccination 
relative to prevaccination).
Empirical reverse cumulative distribution curves (RCDCs) will be presented for each 
toxin A– and toxin B–specific antibody level.
 
.
9.3. Safety Analysis
The safety population will include all subjects who receive at least 1 dose of an 
investigational product. For the safety analysis, subjects will be analyzed according to the 
investigational product received. For each stage (original planned stage [first 3 vaccinations]
and extension stage [fourth vaccination]), all safety data will be analyzed separately for each 
planned dosing regimen (Days 1, 8, and 30 or Months 0, 1, and 6).
AEs will be categorized according to MedDRA and will be summarized by vaccine group. 
All summaries of AEs will show the number and percentage of subjects experiencing at least 
1 event and the number of events for each vaccine group.
The proportion of subjects reporting local reactions at the injection site and systemic events
on any day within the 7- to 14-day period after vaccination will be descriptively summarized 
by vaccine group. Severities of local reactions and systemic events reported after each 
vaccination will also be descriptively summarized by vaccine group. Two-sided exact 95% 
CIs will be presented with the proportions.
.
9.4. Analysis Timing
Six analyses, described below, will be performed for this study.
CCI
CCI

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As there is no formal statistical hypothesis test conducted for the protocol and all statistical 
analyses are descriptive in nature, and the study will not be terminated because of favorable 
results at any of the analyses, there will be no formal multiplicity adjustments applied in all 
analyses. All CIs will be created at nominal level without multiplicity adjustments.
Before the first analysis is initiated, the details of the objectives, decision criteria, and 
method of maintaining the study blind as per Pfizer’s SOPs will be documented and 
approved in an IRC charter and the detailed statistical analysis plan documented and 
approved in an SAP.
9.4.1. First Interim Analysis
The first interim analysis is planned for the study when Month 2 data are available from all 
continuing subjects. All available immunogenicity and safety data will be included in this
analysis.
The objective of the first analysis is to determine the suitability of the Day 1, 8, and 30 
regimen for further study.
9.4.2. Second Interim Analysis
The second interim analysis is planned for the study when Month 7 data are available from 
approximately the first 700 continuing subjects. All available immunogenicity and safety 
data will be included in this analysis.
The objective of the second analysis is to inform internal program-level decision making.
9.4.3. Third Interim Analysis
The third interim analysis is planned for the study when Month 7 data are available from all 
continuing subjects. All available immunogenicity and safety data will be included in this 
analysis.
The objective of the third analysis is to determine the suitability of the Month 0, 1, and 6
regimen for further study and to select the optimal vaccine dose level for further study.
9.4.4. Fourth Interim Analysis
The fourth interim analysis will be conducted when 6 months post–Dose 3 data are available 
from all continuing subjects (Month 7 for subjects receiving the Day 1, 8, and 30 regimen 
and Month 12 for subjects receiving the Month 0, 1, and 6 regimen). All available 
immunogenicity and safety data will be included in this analysis.
The objective of the fourth analysis is to support regulatory interactions.
9.4.5. Analysis for the Primary Clinical Study Report
The analysis for the primary clinical study report (CSR) will be performed when all subjects 
have completed Visit 9 and all immunogenicity and safety data up to and including Visit 9 

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are available. No multiplicity adjustments will be made on the descriptive analyses. All CIs 
will be created at nominal level in this analysis.
9.4.6. Analysis During the Extension Stage
No formal interim analyses are planned for the extension stage. Safety and immunogenicity 
data will be descriptively summarized periodically in order to protect the safety of subjects
and to support internal program-level decision making.
The analysis for the supplemental CSR will be performed when all extension-stage 
immunogenicity and safety data up to Visit 17 are available for subjects who entered the 
extension stage. No multiplicity adjustments will be made on the descriptive analyses. All 
CIs will be created at nominal level in this analysis.
9.5. Data Monitoring Committee
This study will utilize a Pfizer IRC, which will be responsible for reviewing data in the event 
the triggered safety data review rule is met and also to review the results of the 4 planned 
interim analyses. The IRC will be composed of senior Pfizer Vaccine Research & 
Development personnel, not directly involved with the study conduct, which serves to make 
recommendations based on safety findings or the results of the interim analyses. These could 
include advance ordering of future clinical trial supplies or an administrative increase in the 
number of subjects enrolled by a maximum of 30%.
In addition, the study will use an E-DMC independent of Pfizer. The E-DMC will be 
responsible for ongoing monitoring of the safety of subjects in the study, including review of 
unblinded data according to the charter. The E-DMC is scheduled to hold regular review 
meetings approximately twice a year to review safety results. Additional ad hoc E-DMC 
meetings will be held as needed. The recommendations made by the E-DMC to alter the 
conduct of the study will be forwarded to Pfizer for final decision. Pfizer will forward such 
decisions, which may include summaries of aggregate analyses of endpoint events and of 
safety data that are not endpoints, to regulatory authorities, as appropriate.
10. QUALITY CONTROL AND QUALITY ASSURANCE
Pfizer or its agent will conduct periodic monitoring visits during study conduct to ensure that 
the protocol and GCPs are being followed. The monitors may review source documents to 
confirm that the data recorded on CRFs are accurate. The investigator and institution will 
allow Pfizer monitors/auditors or its agents and appropriate regulatory authorities direct 
access to source documents to perform this verification. This verification may also occur 
after study completion.
During study conduct and/or after study completion, the study site may be subject to review 
by the institutional review board (IRB)/ethics committee (EC), and/or to quality assurance 
audits performed by Pfizer, or companies working with or on behalf of Pfizer, and/or to 
inspection by appropriate regulatory authorities.

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The investigator(s) will notify Pfizer or its agents immediately of any regulatory inspection 
notification in relation to the study. Furthermore, the investigator will cooperate with Pfizer 
or its agents to prepare the study site for the inspection and will allow Pfizer or its agent, 
whenever feasible, to be present during the inspection. The investigator will promptly 
provide copies of the inspection findings to Pfizer or its agent.  Before response submission 
to the regulatory authorities, the investigator will provide Pfizer or its agents with an 
opportunity to review and comment on responses to any such findings.
It is important that the investigator(s) and their relevant personnel are available during the 
monitoring visits and possible audits or inspections and that sufficient time is devoted to the 
process.
11. DATA HANDLING AND RECORD KEEPING
11.1. Case Report Forms/Electronic Data Record
As used in this protocol, the term CRF should be understood to refer to either a paper form or 
an electronic data record or both, depending on the data collection method used in this study.
A CRF is required and should be completed for each included subject. The completed 
original CRFs are the sole property of Pfizer and should not be made available in any form to 
third parties, except for authorized representatives of Pfizer or appropriate regulatory 
authorities, without written permission from Pfizer.
The investigator has ultimate responsibility for the collection and reporting of all clinical, 
safety, and laboratory data entered on the CRFs and any other data collection forms (source 
documents) and ensuring that they are accurate, authentic/original, attributable, complete, 
consistent, legible, timely (contemporaneous), enduring, and available when required. The 
CRFs must be signed by the investigator or by an authorized staff member to attest that the 
data contained on the CRFs are true. Any corrections to entries made in the CRFs or source 
documents must be dated, initialed, and explained (if necessary) and should not obscure the 
original entry.
In most cases, the source documents are the hospital's or the physician's subject chart. In 
these cases, data collected on the CRFs must match the data in those charts.
In some cases, the CRF, or part of the CRF, may also serve as source documents. In these 
cases, a document should be available at the investigative site as well as at Pfizer and clearly 
identify those data that will be recorded in the CRF, and for which the CRF will stand as the 
source document.
11.2. Record Retention
To enable evaluations and/or audits from regulatory authorities or Pfizer, the investigator 
agrees to keep records, including the identity of all participating subjects (sufficient 
information to link records, eg, CRFs and hospital records), all original signed informed 
consent documents, copies of all CRFs, safety reporting forms, source documents, and 
detailed records of treatment disposition, and adequate documentation of relevant 

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correspondence (eg, letters, meeting minutes, and telephone call reports). The records should 
be retained by the investigator according to the ICH guidelines, according to local 
regulations, or as specified in the clinical study agreement (CSA), whichever is longer.
If the investigator becomes unable for any reason to continue to retain study records for the 
required period (eg, retirement, relocation), Pfizer should be prospectively notified. The 
study records must be transferred to a designee acceptable to Pfizer, such as another 
investigator, another institution, or an independent third party arranged by Pfizer.
Investigator records must be kept for a minimum of 15 years after completion or 
discontinuation of the study or for longer if required by applicable local regulations.
The investigator must obtain Pfizer's written permission before disposing of any records, 
even if retention requirements have been met.
12. ETHICS
12.1. Institutional Review Board/Ethics Committee
It is the responsibility of the investigator to have prospective approval of the study protocol, 
protocol amendments, informed consent documents, and other relevant documents, eg,
recruitment advertisements, if applicable, from the IRB/EC. All correspondence with the 
IRB/EC should be retained in the investigator file. Copies of IRB/EC approvals should be 
forwarded to Pfizer.
The only circumstance in which an amendment may be initiated prior to IRB/EC approval is 
where the change is necessary to eliminate apparent immediate hazards to the subjects. In 
that event, the investigator must notify the IRB/EC and Pfizer in writing immediately after 
the implementation.
12.2. Ethical Conduct of the Study
The study will be conducted in accordance with legal and regulatory requirements, as well as 
the general principles set forth in the International Ethical Guidelines for Biomedical 
Research Involving Human Subjects (Council for International Organizations of Medical 
Sciences 2002), Guidelines for GCP (ICH 1996), and the Declaration of Helsinki (World 
Medical Association 2008).
In addition, the study will be conducted in accordance with the protocol, the ICH guideline 
on GCP, and applicable local regulatory requirements and laws.
12.3. Subject Information and Consent
All parties will ensure protection of subject personal data and will not include subject names 
or other identifiable data in any reports, publications, or other disclosures, except where
required by law.
When study data are compiled for transfer to Pfizer and other authorized parties, subject 
names, addresses, and other identifiable data will be replaced by a numerical code based on a 
numbering system provided by Pfizer in order to de-identify study subjects. The study site 

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will maintain a confidential list of subjects who participated in the study, linking each 
subject’s numerical code to his or her actual identity. In case of data transfer, Pfizer will 
maintain high standards of confidentiality and protection of subject’s personal data consistent 
with applicable privacy laws.
The informed consent documents must be in compliance with ICH GCP, local regulatory 
requirements, and legal requirements, including applicable privacy laws.
The informed consent documents used during the informed consent process must be 
reviewed and approved by the sponsor, approved by the IRB/EC before use, and available for 
inspection.
The investigator must ensure that each study subject is fully informed about the nature and 
objectives of the study and possible risks associated with participation.
The investigator, or a person designated by the investigator, will obtain written informed 
consent from each subject before any study-specific activity is performed. The investigator 
will retain the original of each subject's signed consent document.
12.4. Subject Recruitment
Advertisements approved by IRBs/ECs and investigator databases may be used as 
recruitment procedures.
Pfizer will have an opportunity to review and approve the content of any study recruitment 
materials directed to potential study subjects before such materials are used.
12.5. Reporting of Safety Issues and Serious Breaches of the Protocol or ICH GCP
In the event of any prohibition or restriction imposed (ie, clinical hold) by an applicable 
competent authority in any area of the world, or if the investigator is aware of any new 
information that might influence the evaluation of the benefits and risks of the investigational 
product, Pfizer should be informed immediately.
In addition, the investigator will inform Pfizer immediately of any urgent safety measures 
taken by the investigator to protect the study subjects against any immediate hazard, and of 
any serious breaches of this protocol or of ICH GCP that the investigator becomes aware of.
13. DEFINITION OF END OF TRIAL
The end of trial in all participating countries is defined as last subject last visit (LSLV), ie,
when the final subject completes Visit 17 in the extension stage.
14. SPONSOR DISCONTINUATION CRITERIA
Premature termination of this study may occur because of a regulatory authority decision, 
change in opinion of the IRB/EC, or investigational product safety problems, or at the 
discretion of Pfizer. In addition, Pfizer retains the right to discontinue development of 
C difficile vaccine at any time.

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If a study is prematurely terminated or discontinued, Pfizer will promptly notify the 
investigator. After notification, the investigator must contact all participating subjects and 
the hospital pharmacy (if applicable) within 2 weeks. As directed by Pfizer, all study 
materials must be collected and all CRFs completed to the greatest extent possible.
15. PUBLICATION OF STUDY RESULTS
15.1. Communication of Results by Pfizer
Pfizer fulfills its commitment to publicly disclose clinical trial results through posting the 
results of studies on www.clinicaltrials.gov (ClinicalTrials.gov), the European Clinical Trials 
Database (EudraCT) and/or www.pfizer.com, and other public registries in accordance with 
applicable local laws/regulations.
In all cases, study results are reported by Pfizer in an objective, accurate, balanced, and 
complete manner and are reported regardless of the outcome of the study or the country in 
which the study was conducted.
www.clinicaltrials.gov
Pfizer posts clinical trial US Basic Results on www.clinicaltrials.gov for Pfizer-sponsored 
interventional studies conducted in patients that evaluate the safety and/or efficacy of a Pfizer 
product, regardless of the geographical location in which the study is conducted.  US Basic 
Results are submitted for posting within 1 year of the primary completion date for studies in 
adult populations or within 6 months of the primary completion date for studies in pediatric 
populations.
Primary completion date is defined as the date that the final subject was examined or 
received an intervention for the purposes of final collection of data for the primary outcome, 
whether the clinical study concluded according to the prespecified protocol or was 
terminated.
 
r 
www.pfizer.com
Pfizer posts Public Disclosure Synopses (clinical study report synopses in which any data 
that could be used to identify individual patients has been removed) on www.pfizer.com for 
Pfizer-sponsored interventional studies at the same time the US Basic Results document is 
posted to www.clinicaltrials.gov.
CCI
CCI

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15.2. Publications by Investigators
Pfizer supports the exercise of academic freedom and has no objection to publication by 
principal investigator of the results of the study based on information collected or generated 
by principal investigator, whether or not the results are favorable to the Pfizer product.
However, to ensure against inadvertent disclosure of confidential information or unprotected 
inventions, the investigator will provide Pfizer an opportunity to review any proposed 
publication or other type of disclosure of the results of the study (collectively, “publication”) 
before it is submitted or otherwise disclosed.
The investigator will provide any publication to Pfizer at least 30 days before they are 
submitted for publication or otherwise disclosed. If any patent action is required to protect 
intellectual property rights, the investigator agrees to delay the disclosure for a period not to 
exceed an additional 60 days.
The investigator will, on request, remove any previously undisclosed confidential 
information before disclosure, except for any study- or Pfizer product-related information 
necessary to the appropriate scientific presentation or understanding of the study results.
If the study is part of a multicenter study, the investigator agrees that the first publication is 
to be a joint publication covering all study sites, and that any subsequent publications by the 
principal investigator will reference that primary publication. However, if a joint manuscript 
has not been submitted for publication within 12 months of completion or termination of the 
study at all participating sites, the investigator is free to publish separately, subject to the 
other requirements of this section.
For all publications relating to the study, the institution will comply with recognized ethical 
standards concerning publications and authorship, including Section II - “Ethical 
Considerations in the Conduct and Reporting of Research” of the Uniform Requirements for 
Manuscripts Submitted to Biomedical Journals, http://www.icmje.org/index.html#authorship, 
established by the International Committee of Medical Journal Editors.
Publication of study results is also provided for in the CSA between Pfizer and the 
institution. In this section entitled Publications by Investigators, the defined terms shall have 
the meanings given to them in the CSA.
If there is any conflict between the CSA and any attachments to it, the terms of the CSA 
control.  If there is any conflict between this protocol and the CSA, this protocol will control 
as to any issue regarding treatment of study subjects, and the CSA will control as to all other 
issues.

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16. REFERENCES
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Dubberke E. Clostridium difficile infection: the scope of the problem. J Hosp Med 
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Loo VG, Poirier L, Miller MA, et al. A predominantly clonal multi-institutional 
outbreak of Clostridium difficile-associated diarrhea with high morbidity and mortality. 
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McDonald LC, Killgore GE, Thompson A, et al. An epidemic, toxin gene-variant strain 
of Clostridium difficile. N Engl J Med 2005;353(23):2433-41.
8.
Pepin J, Valiquette L, Alary ME, et al. Clostridium difficile-associated diarrhea in a 
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9.
Gerding DN. Clindamycin, cephalosporins, fluoroquinolones, and Clostridium difficile-
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Kyne L, Farrell RJ, Kelly CP. Clostridium difficile. Gastroenterol Clin North Am 
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Sunenshine RH, McDonald LC. Clostridium difficile-associated disease: new challenges 
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Bartlett JG, Chang TW, Gurwith M, et al. Antibiotic-associated pseudomembranous 
colitis due to toxin-producing clostridia. N Engl J Med 1978;298(10):531-4.
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Bartlett JG. Clinical practice. Antibiotic-associated diarrhea. N Engl J Med
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Gerding DN, Muto CA, Owens RC Jr. Treatment of Clostridium difficile infection. Clin 
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15.
Louie TJ, Miller MA, Mullane KM, et al. Fidaxomicin versus vancomycin for 
Clostridium difficile infection. N Engl J Med 2011;364(5):422-31.
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Mattila E, Uusitalo-Seppala R, Wuorela M, et al. Fecal transplantation, through 
colonoscopy, is effective therapy for recurrent Clostridium difficile infection. 
Gastroenterology 2012;142(3):490-6.
17.
van Nood E, Vrieze A, Nieuwdorp M, et al. Duodenal infusion of donor feces for 
recurrent Clostridium difficile. N Engl J Med 2013;368(5):407-15.
18.
Anonymous. Guidance for industry: enforcement policy regarding investigational new 
drug requirements for use of fecal microbiota for transplantation to treat Clostridium 
difficile infection not responsive to standard therapies. 2013. Available: 
http://www.fda.gov/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformati
on/Guidances/Vaccines/ucm361379.htm. 24 Jul 2013.
19.
Kyne L, Warny M, Qamar A, et al. Asymptomatic carriage of Clostridium difficile and 
serum levels of IgG antibody against toxin A. N Engl J Med 2000;342(6):390-7.
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Kyne L, Warny M, Qamar A, et al. Association between antibody response to toxin A 
and protection against recurrent Clostridium difficile diarrhoea. Lancet 
2001;357(9251):189-93.
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Warny M, Vaerman JP, Avesani V, et al. Human antibody response to Clostridium 
difficile toxin A in relation to clinical course of infection. Infect Immun 
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Babcock GJ, Broering TJ, Hernandez HJ, et al. Human monoclonal antibodies directed 
against toxins A and B prevent Clostridium difficile-induced mortality in hamsters. 
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Giannasca PJ, Zhang ZX, Lei WD, et al. Serum antitoxin antibodies mediate systemic 
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Torres JF, Lyerly DM, Hill JE, et al. Evaluation of formalin-inactivated Clostridium 
difficile vaccines administered by parenteral and mucosal routes of immunization in 
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Lowy I, Molrine DC, Leav BA, et al. Treatment with monoclonal antibodies against 
Clostridium difficile toxins. N Engl J Med 2010;362(3):197-205.
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Gupta SB, Mehta V, Herring TA, et al. A large prospective North American 
epidemiologic study of hospital-associated Clostridium difficile colonization & 
infection. In: Rupnik M, Janezic S, editors. 4th ICDS - International Clostridium difficile
Symposium abstract book; 20-22 Sep 2012; Bled, Slovenia. Maribor, Slovenia: Institute 
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27.
Bartlett JG, Gerding DN. Clinical recognition and diagnosis of Clostridium difficile
infection. Clin Infect Dis 2008;46(Suppl 1):S12-8.
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Bauer MP, Notermans DW, van Benthem BH, et al. Clostridium difficile infection in 
Europe: a hospital-based survey. Lancet 2011;377(9759):63-73.
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McGowan AP, Lalayiannis LC, Sarma JB, et al. Thirty-day mortality of Clostridium 
difficile infection in a UK National Health Service Foundation Trust between 2002 and 
2008. J Hosp Infect 2010;77(1):11-5.
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Simor AE. Diagnosis, management, and prevention of Clostridium difficile infection in 
long-term care facilities: a review. J Am Geriatr Soc 2010;58(8):1556-64.
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Foglia G, Shah S, Luxemburger C, et al. Clostridium difficile: development of a novel 
candidate vaccine. Vaccine 2012;30(29):4307-9.
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ClinicalTrials.gov. Study of a candidate Clostridium difficile toxoid vaccine in subjects 
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33.
Valneva. Clostridium difficile. Available: 
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18_Valneva_C_diff_Phase_II_PR_EN.pdf. 18 Dec 2014.
34.
Johnson JE, Megati S, Roopchand VS, et al. A novel approach to a C. difficile toxoid 
vaccine: immunogenicity and preclinical efficacy. In: Rupnik M, Janezic S, editors. 4th
ICDS - International Clostridium difficile Symposium abstract book; 20-22 Sep 2012; 
Bled, Slovenia. Maribor, Slovenia: Institute of Public Health; 2012: p. 30.
35.
Report GR325. 5-Week intramuscular injection toxicity study with PF-06425090 in 
rabbits with a 4-week recovery phase. 18 Jun 2012.
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Appendix 1. Abbreviations
Abbreviation
Term
AE
adverse event
ALT
alanine aminotransferase
AST
aspartate aminotransferase
C difficile
Clostridium difficile
CBER
Center for Biologics Evaluation and Research
CDAD
C difficile–associated diarrhea 
CI
confidence interval 
COPD
chronic obstructive pulmonary disease
CRF
case report form
CSA
clinical study agreement
CSR
clinical study report
DAI
dosage and administration instructions
DMARD
disease-modifying antirheumatic drug 
EC
ethics committee
e-diary
electronic diary
E-DMC
external data monitoring committee
EDP
exposure during pregnancy
ELISA
enzyme-linked immunosorbent assay
EU
European Union
EudraCT
European Clinical Trials Database
FDA
Food and Drug Administration (United States)
FIH
first-in-human
GCP
Good Clinical Practice
GMC
geometric mean concentration
GMFR
geometric mean fold rise
HBV
hepatitis B virus 
HCV
hepatitis C virus
HIV
human immunodeficiency virus 
IB
investigator’s brochure
IC50
50% inhibitory concentration
ICD
informed consent document
ICH
International Council for Harmonisation
IgG
immunoglobulin G
IND
investigational new drug application
INR
international normalized ratio
IRB
institutional review board
IRC
internal review committee
IRT
interactive response technology
IUD
intrauterine device

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Abbreviation
Term
LFT
liver function test
LLOQ
lower limit of quantitation
LOD
limit of detection 
LSLV
last subject last visit 
mAb
monoclonal antibody
MedDRA
Medical Dictionary for Regulatory Activities
mITT
modified intent-to-treat
N/A
not applicable
NHP
nonhuman primate
PT
preferred term
RCDC
reverse cumulative distribution curve 
RRI
research-related injury
SAE
serious adverse event
SAP
statistical analysis plan
SD
standard deviation 
SOP
standard operating procedure
SRM
study reference manual
SRSD
single reference safety document
TcdA
C difficile toxin A
TcdB
C difficile toxin B
ULN
upper limit of normal
US
United States

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