Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
Vaccine Name and Compound Number: Clostridioides difficile Vaccine, Compound
Number: PF 06425090
Report Title: Final Report: A Phase 3, Placebo-Controlled, Randomized, Observer-Blinded
Study to Evaluate the Efficacy, Safety, and Tolerability of a Clostridium Difficile Vaccine inAdults 50 Years of Age and Older
Protocol Number: B5091007
Sponsor: Pfizer Inc.
Phase of Development: Phase 3
First Subject First Visit: 29 March 2017
Last Subject Last Visit: 21 December 2021
Coordinating Investigator(s):, MDLouis Stokes Cleveland VA Medical Center
10701 East Boulevard,Cleveland, Ohio, 44106
United States

Study Center(s): This study was conducted at 410 sites in multiple countries. Refer toAppendix 16.1.4.1 for a list of sites involved in this study.
Date of Current Version: 16 August 2022
Date(s) of Previous Report(s): 27 July 2022

PFIZER CONFIDENTIAL
Page 1

Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSISOBJECTIVES
Study Objectives and Endpoints:
Clostridioides difficile infection (CDI) case definitions referred to in Table S1 are describedin Appendix 1.
Table S1. Study Objectives and Endpoints
Primary Efficacy Objective(s):Primary Efficacy Endpoint(s):To demonstrate that Pfizer’s Clostridium difficile(C difficile) vaccine is effective in reducing the incidenceof a first primary episode of CDI (case definition 1).a

CDI incidence per 1000 person-years of follow-up,
assessed during up to 2 time periods (each analysis
was performed only if the preceding one wassuccessful):After receipt of the third dose ofinvestigational product onwards.

After receipt of the second dose ofinvestigational product onwards.

Primary Safety Objective(s):Primary Safety Endpoint(s):Local reactions (pain, erythema, and induration), as
self-reported on electronic diaries (e-diaries) for up
to 7 days following each dose of investigational
product.

Systemic events (fever, vomiting, headache, fatigue,new or worsening muscle pain, and new orworsening joint pain), as self-reported on e-diaries
for up to 7 days following each dose ofinvestigational product.

Nonserious AEs from the signing of informed
consent document (ICD) to 1 month after receipt ofthe third dose of investigational product.

SAEs from the signing of the ICD to 6 months after
receipt of third dose of investigational product.

To evaluate the safety profile of Pfizer’s C difficile
vaccine as measured by the percentage of participants
reporting local reactions and systemic events, adverse
events (AEs), and serious adverse events (SAEs).

Evaluation

Secondary Objective(s):Order

3-Dose Family

1.

Secondary Endpoint(s):To evaluate the efficacy of Pfizer’sC difficile vaccine in reducing:The incidence of all CDI cases (casedefinition 1 and case definition 2).

CDI incidence per 1000 person-years of follow-up,
assessed after receipt of the third dose ofinvestigational product onwards.

PFIZER CONFIDENTIAL
Page 2

Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
1. Evidence of a personally signed and dated ICD indicating that the participant wasinformed of all pertinent aspects of the study.
2. Willing and able to comply with scheduled visits, vaccination plan, and other study
procedures.
3. 50 Years or older at enrollment.
4. Participants with an increased risk of future contact with healthcare systems by virtue of:At least 1 inpatient hospitalization ≥2 nights’ duration in the previous 12 months;orAt least 2 emergency room visits in the previous 12 months; orAt least 10 outpatient visits (primary and/or secondary care visits; defined as an inperson visit to the office/clinic of a prescribing healthcare provider for the purposesof the diagnosis, treatment, or ongoing management of a medical condition, excluding
pharmacy and mental health visits) in the previous 12 months; orResidence in a skilled nursing facility (a residential institution that provides
professional nursing care and rehabilitation services, usually following discharge
from the hospital); orResidence in a nursing home (a residential institution that provides assistance withactivities of daily living); orInpatient hospitalization ≥2 nights’ duration scheduled ≥37 days after randomization.

Or participants who received systemic (ie, oral or injected) antibiotics for a minimumof 48 hours at any time in the previous 12 weeks.

5. Ability to be contacted by telephone during study participation.
6. Negative urine pregnancy test for female participants of childbearing potential.
Female participants of nonchildbearing potential must had met at least 1 of the following
criteria:a. Achieved postmenopausal status, defined as follows: cessation of regular menses forat least 12 consecutive months with no alternative pathological or physiologicalcause; status may be confirmed with a serum follicle-stimulating hormone (FSH)level confirming the postmenopausal state
PFIZER CONFIDENTIAL
Page 6

Final Clinical Study Report
Protocol B5091007CLINICAL STUDY REPORT SYNOPSIS
b. Had undergone a documented hysterectomy and/or bilateral oophorectomy
c. Had medically confirmed ovarian failure
All other female participants (including female participants with tubal ligations) wereconsidered to be of childbearing potential.
Exclusion Criteria
Exclusion criteria were as follows:
1. Investigator site staff members directly involved in the conduct of the study and theirfamily members, site staff members otherwise supervised by the investigator, orparticipants who were Pfizer employees, including their family members, directly
involved in the conduct of the study.
2. Participation in other studies involving investigational drug(s)/vaccine(s) within 28 days
prior to study entry until Visit 5 (1 month after the third vaccination).
3. Previous administration of an investigational C difficile vaccine or C difficile monoclonal
antibody therapy.
4. Prior episode of CDI, confirmed by either laboratory test or diagnosis of
pseudomembranous colitis at colonoscopy, at surgery, or histopathologically.
5. Receipt of blood products or immunoglobulins within 6 months before enrollment.
6. Participants who were unable to respond to vaccination because of:Metastatic malignancy; orEnd-stage renal disease (glomerular filtration rate <15 mL/min/1.73 m2 or on dialysis); or
Any serious medical disorder that in the investigator’s opinion was likely to be fatal within
the next 12 months; orCongenital or acquired immunodeficiency; orReceipt of systemic corticosteroids (≥20 mg/day of prednisone or equivalent) for ≥14 days
within 28 days of enrollment; orReceipt of chronic systemic treatment with other known immunosuppressant medications, orradiotherapy, within 6 months of enrollment.
7. Known infection with human immunodeficiency virus.
PFIZER CONFIDENTIAL
Page 7

Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
8. Any bleeding disorder or anticoagulant therapy that would contraindicate intramuscularinjection.
9. Any contraindication to vaccination or vaccine components, including previous
anaphylactic reaction to any vaccine or vaccine-related components.
10. Prior small- or large-bowel resection (did not include appendectomy).
11. Any condition or treatment resulting in frequent diarrhea (≥3 loose stools per day morethan once per month), as reported by the participant.
12. Other acute or chronic medical or psychiatric condition including recent (within the pastyear) or active suicidal ideation or behavioral or laboratory abnormality that may haveincreased the risk associated with study participation or investigational productadministration or may have interfered with the interpretation of study results and, in the
judgment of the investigator, would make the participant inappropriate for entry into thisstudy.
13. Pregnant female participants; breastfeeding female participants; fertile male participants
and female participants of childbearing potential who were, in the opinion of theinvestigator, sexually active and at risk for pregnancy with their partner(s) and wereunwilling or unable to use 2 highly effective methods of contraception as outlined in thisprotocol from the signing of the informed consent until at least 28 days after the last doseof investigational product.
Vaccines Administered: All injections were administered in the upper deltoid muscle,
preferably of the nondominant arm, by the unblinded administrator. Participants received
1 dose of C difficile vaccine/placebo at Visits 1 (Month 0), 2 (Month 1), and 4 (Month 6).
A list of the investigational products administered in this study and their respective lot
numbers is provided in Table S2.
Table S2. Investigational Product Lot Numbers
Vendor Lot Number(Manufacturer)

Lot Numbera(Pfizer)

Investigational Product

Manufacturer

Clostridium difficile vaccine

Pfizer

R5600016-005448

Aluminum hydroxide diluent(1 mg/mL)

Pfizer

R5256416-005447

Placebo (0.9% sodium chloride)

Pfizer

R3015916-005048

a. Lot number assigned to the investigational product by Pfizer Global Clinical Supply.
Protocol B5091007 Investigational Product Lot Numbers Table – Final, Version 1.0, 13Oct2021.

PFIZER CONFIDENTIAL
Page 8Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
Efficacy Evaluations: Any time during the study period that a subject experienced passageof 3 or more unformed stools (Bristol stool chart types 5-7) within 24 hours, the subject wasasked to collect the third or subsequent stool for provision to the central laboratory. Thesubject was provided with a sample collection kit and instructed on how to package thesample. The sample was shipped to a processing laboratory. The processing laboratory
prepared aliquots of the stool for later analysis at the central laboratory.
Safety Evaluations: Participants recorded local reactions (redness, swelling, and pain at theinjection site) and systemic events (vomiting, headache, fatigue, new or worsening muscle
pain, and new or worsening joint pain) using an e-diary for 7 days following vaccination,
which allowed recording of these assessments only within a fixed time window and provided
an accurate representation of the participant’s experience at that time.
AEs and SAEs were collected from the signing of the ICD through and including Visit 5(Month 7) for AEs and through Visit 6 (Month 12) for SAEs.
Statistical Methods: The primary efficacy hypotheses were tested for null hypothesis
vaccine efficacy (VE) ≤20% by a fixed sequence testing procedure, with a testing order ofthe first primary CDI cases (Case Definition 1) occurring from 14 days after receipt of thethird dose, followed by those occurring from 14 days after receipt of the second dose.
Secondary objectives for vaccine efficacy were evaluated with hypothesis tests sequentiallywith the order specified in Table S1. After accounting for multiplicity due to the interim andfinal analyses, as well as multiple endpoints, the type I error for the final analyses wasadjusted to α of 0.018 (1-sided) for the primary efficacy endpoints and α of 0.009 (1-sided)
for the secondary endpoints. Unadjusted α of 0.025 (1-sided) was used for exploratory
endpoints. The corresponding 2-sided 96.4% confidence intervals (CIs), 98.2% CIs, and 95%
CIs were presented for primary, secondary, and exploratory endpoints respectively. A null
hypothesis would be rejected and the VE for the corresponding efficacy endpoint would bedemonstrated if the lower bound of the 2-sided confidence interval for the VE >20%.
For endpoints involving the identification of 1 case per participant, the VE was defined as apercentage of VE = 100 × (1 – infection rate ratio [IRR]), where IRR was the infection rateratio adjusted for surveillance time between the C difficile vaccine group and the placebogroup. The corresponding VE confidence intervals were derived using the Clopper-Pearson
method adjusted for surveillance time.
For endpoints involving the identification of 1 or more cases per participant, such as all
episodes of CDI as defined by case definitions 1 and 2, VE and the corresponding confidence
intervals were computed using a proportional means model.
The proportion of participants reporting local reactions at the injection site and systemic
events on any day within the 7-day period after vaccination were descriptively summarizedby vaccine group. Severities of local reactions and systemic events reported after eachvaccination were also descriptively summarized by vaccine group. Two-sided 95% CIs basedon the Clopper Pearson method were presented with the proportions.
PFIZER CONFIDENTIAL
Page 9Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
AEs were categorized according to Medical Dictionary for Regulatory Activities andsummarized by vaccine group. All summaries of AEs showed the number and percentage ofparticipants experiencing at least 1 event and the number of events for each vaccine group.
Two-sided 95% CIs based on the Clopper-Pearson method were presented for (S)AEpercentages. AEs were summarized using 3-tier methodology.
RESULTS
Subject Disposition and Demography:
A total of 17,535 participants were randomized to the study (8766 to the C difficile group and8769 to the placebo group). Withdrawal by subject (~24% for each group) was the mostfrequent reason for withdrawal. Overall, 7894 (90.1%) participants in the C difficile groupand 7967 (90.9%) participants in the placebo group completed the vaccination schedule (all3 doses), and a total of 5533 (63.1%) participants in the C difficile group and 5574 (63.6%) in
the placebo group completed the study.
The 2 vaccine groups in the safety population were generally similar with respect todemographic characteristics. Overall, 51.5% participants were female and 48.5% were male.
The majority were White (79.2%) and from North America (65.1%); the mean age at
randomization was 68 years. The 2 vaccine groups were also similar with regards to Charlson
Comorbidity Index scores at baseline, with most participants scoring 0 (59.9% in theC difficile group and 59.7% in the placebo group) or 1 (25.9% in the C difficile group and26.9% in the placebo group).
Efficacy Results:
Primary Endpoints
The vaccine efficacy of C difficile vaccine did not meet the primary objective for thereduction in the incidence of first episodes of primary CDI 14 days after receipt of the third
dose of investigational product onwards: estimated VE of 31.0% with the lower bound of the2-sided 96.4% CI being -38.7%, ≤ 20% (success criterion). Since the study failed todemonstrate vaccine efficacy for the first primary endpoint, hypothesis tests for all thesubsequent vaccine efficacy endpoints were not performed per the pre-specified statisticalanalysis procedure. Estimates and associated 2-sided CIs for all the other efficacy endpoints
were described.
The estimated vaccine efficacy for the second primary endpoint of reduction of primary CDI
after receipt of the second dose of investigational product was 28.6% with the lower bound of
the 2-sided 96.4% CI being -28.4%.

PFIZER CONFIDENTIAL
Page 10Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
Secondary Endpoints
In the 3-dose family, the first objective of reduction of the incidence of all CDI (CaseDefinitions 1 and 2) and the third objective of reduction of the incidence of recurrent CDI(Case Definition 2) were not met.
Although the primary endpoints did not meet success criteria, positive results were observedin the reduction of severity of CDI, as measured by reduction in duration of CDI episodes aswell as the requirement to seek medical attention. Due to an outlier CDI episode with anextremely long duration in the placebo group, the median and the non-parametric Wilcoxon
rank sum test were more appropriate statistics to compare the distributions of durations
between the two groups. The median time to resolution of diarrhea in participants with a firstprimary episode of CDI ≥14 days after receipt of the third dose was 1 day in the C difficile
group and 4 days in the placebo group, with a nominal Wilcoxon rank sum 2-sided p-value of0.0172. This translates to a 75% reduction in median duration of disease episode. Duration ofCDI episodes is a predictor of medically attended CDI. In a post hoc analysis of first primaryepisodes of CDI occurring at least 14 days after Dose 3 in the placebo group, the median
duration of CDI episodes that were medically attended was 19 days compared with 2.5 days
for CDI episodes for which medical attention was not sought, with a 2-sided p-value of0.0024 based on Wilcoxon rank sum test.
For those participants with primary episodes of CDI, all participants that sought medical
attention for their episodes were in the placebo group (11 cases in placebo), with an observed
100% VE (lower bound of the 2-sided 95% CI 59.6%) in a post hoc analysis. This indicatesthat the need for medical attention was completely reduced after C difficile vaccination, withprotection extended from ≥14 days after Dose 3 for approximately 4 years.
In an additional post-hoc analysis, all participants that required antibiotics to treat their CDI
episodes were all in the placebo group (10 cases in placebo), with an estimated VE of 100%(lower bound of the 2-sided 95% CI 54.8%), indicating that the C difficile vaccine
significantly reduced antibiotic use in the treatment of first primary episodes of CDI.
Safety Results:
The proportions of participants who reported local reactions within 7 days after each doseand after any dose by maximum severity were similar.
The proportions of participants who reported any related AE from Dose 1 to 1 month after
Dose 3 were 3.4% in the C difficile group compared to 1.7% in the placebo group. The
difference relates to reactogenicity events that were also reported as adverse events seen
mostly as nausea, injection site pain, redness and swelling, arthralgia, myalgia, and headache.
The most commonly reported related system organ class (SOC) was general disorders andadministration site conditions. Related AEs reported in ≥1% of participants within 1 monthafter vaccination were generally similar in both vaccine groups for each of the subgroup
analyses by age group, race, ethnicity, gender, and region
PFIZER CONFIDENTIAL
Page 11

Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
Tier 2 AEs with incidence rate of at least 1% in any vaccine group from Dose 1 to 1 monthafter Dose 3 were most commonly reported in the SOCs of gastrointestinal disorders,infections and infestations, injury, poisoning and procedural complications, musculoskeletaland connective tissue disorders, nervous system disorders, respiratory, thoracic, andmediastinal disorders, and vascular disorders in the C difficile group. Falls were the mostfrequently reported AE in both groups. Nine hundred and eighteen AEs (10.5%) in theC difficile group were assessed as severe and potentially life threatening from the day ofDose 1 to 6 months after Dose 3.
The proportions of participants reporting any SAEs were low and similar (≤12.8%) in theC difficile and placebo groups from Dose 1 to 6 months after Dose 3, and <0.1% in both theC difficile and placebo groups for related SAEs.
The proportion of participants who experienced death from all causes was similar in bothvaccine groups.
The proportion of participants who experienced diarrhea from all causes was similar in bothvaccine groups.
Conclusions: Overall, Study B5091007 enrolled approximately 17,500 adults, 50 years ofage and older. The study was expected to accrue 66 cases of CDI within 2 years of theprimary vaccination series. Due to significant operational challenges created in part by theCOVID-19 pandemic, the final analysis was performed at 42 cases within 4 years after
agreement from the Food and Drug Administration (United States) to amend the protocol.
Although the vaccine did not have the expected impact on all primary CDI cases detected inthis study, it was highly effective at preventing cases severe enough to require medicalintervention. Importantly, outside the clinical trial setting, testing for C difficile must berequested by a healthcare professional, and CDI cases that are too mild to require medical
attention are therefore never diagnosed or reported. Thus, the cases reported in surveillanceand epidemiological studies are all, by definition, medically attended. Despite the study
missing its primary endpoints, based on relevant secondary and post-hoc endpoints, theC difficile vaccine has the potential to provide major public health benefit by reducing theburden of CDI severe enough to require medical attention and associated antibiotic use.
The safety profile in over 8,700 participants who received a least one dose of Pfizer’s
C difficile vaccine was very well tolerated and showed a favorable safety profile.

PFIZER CONFIDENTIAL
Page 12Final Clinical Study Report
Protocol B5091007
CLINICAL STUDY REPORT SYNOPSIS
Appendix 1. CDI Case Definitions
For all definitions, an episode will be considered to have resolved once there have been atleast 2 days without passage of 3 or more unformed stools (Bristol stool chart types 5-7) andthere is no further need for antibiotic treatment for CDI.
Definition 1 – Primary EpisodeA primary episode of CDI (ie, no previous CDI onset in the prior 8 weeks) is defined aseither:
Presence of diarrhea, defined as passage of 3 or more unformed stools (Bristol stool charttypes 5-7) in 24 or fewer consecutive hours; andStool sample that is positive for the toxin B gene (by PCR) and positive for toxin A and/or
toxin B, as measured in the central laboratory.
or:Pseudomembranous colitis diagnosed at colonoscopy, at surgery, or histopathologically;
and corresponding stool sample that is positive for the toxin B gene (via PCR) as
measured in the central laboratory.

Definition 2 – Recurrent EpisodeAn episode of CDI that occurs 8 weeks or less after the onset of a previous CDI episode(provided the symptoms of the previous episode had resolved), defined as either:
Presence of diarrhea, defined as passage of 3 or more unformed stools (Bristol stool charttypes 5-7) in 24 or fewer consecutive hours; andStool sample that is positive for the toxin B gene (by PCR) and positive for toxin A and/ortoxin B, as measured in the central laboratory.
or:
Pseudomembranous colitis diagnosed at colonoscopy, at surgery, or histopathologically; andcorresponding stool sample that is positive for the toxin B gene (by PCR) as measured inthe central laboratory.

PFIZER CONFIDENTIAL
Page 13