Comparative Effectiveness of Adjunctive Clindamycin Versus Linezolid for β-lactam Treated Patients With Invasive Group A Streptococcal Infections: A Target Trial Emulation
Conditions: Streptococcal Sepsis, Invasive Group A Beta-Haemolytic Streptococcal Disease, Necrotizing Soft Tissue Infection, Infection, Bloodstream, Infection, Bacterial, Streptococcal Toxic Shock Syndrome
Sponsor: National Institutes of Health Clinical Center (CC)

Title: Comparative effectiveness of adjunctive clindamycin versus linezolid for β-lactam treated 
patients with invasive Group A Streptococcal infections: A target trial emulation 
 
Protocol ID: BD022389 
 
Date: 11/08/2023 
 
 
 

Significance and Background  
 
Despite advances in supportive care, source control, and antibiotics, mortality from 
invasive Group A streptococcus (GAS), remains high 1-3.  Clindamycin, a protein synthesis 
inhibitor with activity during the stationary phase of bacterial growth has been shown to 
decrease the expression and production of GAS virulence factors and exotoxins 4,5. The addition 
of adjunctive clindamycin therapy to β-lactams is recommended for the management of severe 
invasive GAS and associated necrotizing soft tissue infection (NSTI)6,7. These 
recommendations were initially made based on clindamycin’s anti-toxin effect and animal 
models, which demonstrated reduced efficacy of penicillin compared to clindamycin due to an 
inoculum effect 8. Recently, the preponderance of observational studies has shown a benefit or 
trend towards benefit with the use of adjunctive clindamycin in invasive GAS infections 3,9-11 .  
     
Linezolid is an oxazolidinone antibiotic with a broad spectrum of activity against Gram-
positive organisms. Similar to clindamycin, linezolid inhibits toxin and virulence factor production 
by inhibiting protein synthesis12. Concerns about rising rates of clindamycin resistance among 
BHS 13 have called into question whether linezolid should remain the adjunctive protein 
synthesis inhibitor of choice in BHS infections in lieu of clindamycin 14. However, clinical 
experience to date is limited.   
The severe life-threatening nature of invasive GAS has made recruitment to invasive 
GAS specific clinical trials difficult due to low numbers and a lack of clinician willingness to 
randomize to perceived inferior treatments15. The global increase in invasive GAS cases 16,17 
coupled with the ongoing poor outcomes of these infections has led to many within the ID 
community to call for a re-assessment the feasibility of conducting appropriately powered 
interventional clinical trials 18. Hence, this study aims to emulate a hypothetical target pragmatic 
multi-center, non-blinded trial of adult inpatients in the PINC AITM  dataset with B-lactam treated 
culture confirmed monomicrobial invasive Group A streptococcus (GAS) between the years 
2015-2021. 
 
 

Table 1. Summary of Key Elements of Target Trial and Emulation Trial Components 
 
Protocol 
Component 
Description under target trial  
Description under Emulation  
Eligibility 
Criteria 
Adult inpatients receiving B-lactam 
therapy for culture confirmed 
monomicrobial invasive GAS 
between who can be randomized 
within 72h of index culture 
Adult inpatients receiving B-lactam 
therapy for culture confirmed 
monomicrobial invasive GAS 
between the years 2015-2021 in the 
PINC AITM  dataset who receive anti-
toxin within 3 days of index culture. 
Treatment 
strategies  
B-lactam therapy as primary therapy 
for GAS started within (+/-) 3 days of 
index GAS culture and continued for 
a minimum of 3 consecutive days. 
Adjunctive anti-toxin (clindamycin vs. 
linezolid) therapy started within (+) 3 
days of culture.  
 
 
Same as target trial. 
Assignment 
procedures 
Patients will be randomly assigned 
within 3 days of culture (grace period) 
to clindamycin vs. linezolid. 
Investigators and patients and will be 
aware of the strategy to which they 
have been assigned (pragmatic, 
unblinded, open-label non-inferiority).   
 
Switching between linezolid and 
clindamycin will not be permitted 
under the protocol. 
Patients will be assigned to the anti-
toxin therapy compatible with their 
observed treatment. Difference in 
baseline characteristics and 
confounding by indication will be 
addressed through the creation of a 
propensity weighted cohort.   
 
Those who received both agents 
within the eligibility period will be 
excluded.  
Follow up 
period 
Starts at time of randomization (anti-
toxin initiation) and ends at death, 
discharge to hospice, loss to follow 
up or 90 days, whichever occurs 
earliest. 
Starts at time of anti-toxin initiation 
and ends at death, discharge to 
hospice or discharge from the 
hospital to home or facility or 
administrative censorship on Jan 31st 
2022.  
Outcome 
Primary outcome: Time to all-cause 
mortality (or discharge to hospice) 
censored at 90 days.  
 
Primary outcome: All-cause in-
hospital mortality (or discharge to 
hospice)/ time to death (days).  
Secondary outcome: infection 
attributable 90-day mortality, all-
cause in-hospital mortality, 90-day 
disease reoccurrence, length of stay 
following anti-toxin administration 
among survivors, occurrence of CDI 
Secondary outcome: Length of stay 
following anti-toxin administration 
among survivors, occurrence of CDI 
requiring treatment following anti-
toxin administration.  
 

requiring treatment following anti-
toxin administration up to 90 days, 
occurrence of serotonin surge 
syndrome while on therapy. 
 
Causal 
contrast of 
interest  
Intention-to-treat effect; per-protocol 
effect.  
Observational analog to Intention-to-
treat effect and per-protocol effect. 
Analysis 
plan 
Intention-to-treat and per protocol 
effects estimated via comparison of 
adjusted 90 day in-hospital mortality 
risk assigned to each treatment 
strategy.   
  
Intention-to-treat and per protocol 
effects estimated via comparison of 
adjusted in-hospital mortality risk 
assigned to each treatment strategy.  
 
To account for confounding by 
indication a propensity score (PS) will 
be generated based on the probability 
of receiving linezolid using the patient 
and center level variables selected a 
priori based on clinical judgment, . 
Overlap weighting on a propensity-
score between the two groups will be 
performed with downstream 
adjustment for ICU stay , shock and 
debridement/source control. 
  
Abbreviations: CDI: Clostridioides difficile infection; GAS: Group A streptococcus.  
 
 

 
The study was designed in accordance with the trial emulation approach described by Hernán 
and Robins 19.  The study aims to emulate a hypothetical target pragmatic multi-center, non-
blinded trial. 
 
Eligibility  
Adult inpatients patients (=> 18 y of age) with culture confirmed proven or probable (see below) 
invasive monomicrobial GAS in the PINC AITM  database between 2015-2021 who receive a β-
lactam for a minimum of 3 days within 3 days of culture will be eligible for inclusion in the 
emulated trial. Patient will be included if they then receive anti-toxin therapy within 3 days of 
culture being drawn, as long as B-lactam therapy has been initiated prior or on the same day as 
antitoxin therapy.   
 
Proven and probable invasive GAS definitions will be adapted from the CDC ABCs case 
definition (https://www.cdc.gov/abcs/methodology/case-def-ascertain.html) using a previously 
used algorithm compatible with EHR data which incorporates ICD-codes3 . Proven invasive 
GAS infection will be defined as GAS isolated from a normally sterile site, such as blood, 
cerebrospinal fluid, pleural fluid, peritoneal fluid, pericardial fluid, bone, joint/synovial fluid, or 
internal body site (e.g., lymph node, brain) OR GAS isolated from a wound culture and 
accompanied by necrotizing fasciitis or streptococcal toxic shock syndrome. Probable invasive 
GAS infection is defined as the isolation of GAS from a non-sterile site in patients with either an 
ICD-10 code that matches the clinical infection syndrome (eg ICD-10 for lower respiratory 
infection and BAL culture with GAS) or a non-specific streptococcal infection code. These 
include lower respiratory, musculoskeletal, genitourinary and/or skin and soft tissue samples as 
well other miscellaneous deep-seated infections (eg retropharyngeal abscess). Previously 
identified ICD-9 codes3 will be cross walked to ICD-10 codes manually with additional manual 
de-novo search for compatible ICD-10 codes. Final codes and culture site classification will be 
adjudicated by two infectious diseases clinicians. 
 
Inclusion Criteria 
- 
Adult inpatients patients (=> 18 y of age) 
- 
Monomicrobial Group A streptococcus invasive infection 
- 
Primary therapy with a B-lactam agent (initiated before or on same day as adjunctive 
anti-toxin therapy) 
 
Exclusion Criteria 
Patient will be excluded if they have any of the following : 
• 
Patients with a polymicrobial GAS culture  
• 
Patients who allocated to the linezolid arm but have a documented linezolid resistant 
isolate  
• 
Patient with concomitant MSSA/MRSA invasive infection (+/- seven days of index GAS 
eligibility culture) 
• 
Patients who receive both anti-toxin agents (violation of protocol)  
• 
Patient who don’t complete at least 3 days of B-lactam (violation of protocol) 
 
Primary Outcome measure: 
in-hospital mortality defined as death during hospitalization or discharge to hospice. 
 
Secondary outcome measures: 
• 
Length of stay among survivors  

• 
Clostridioides difficile infection 
o Description: C. difficile positive (PCR or antigen) result within same encounter 
downstream of the antitoxin therapy within 30 days and/or presence of a non 
present on admission (POA) C. difficile diagnosis code in conjugation with receipt 
of C. difficile therapy (PO/rectal vancomycin or PO fidaxomicin or IV 
metronidazole) 
 
 
 
Statistical Analysis Plan 
To account for confounding by indication a propensity score (PS) will be generated based on the 
probability of receiving linezolid using the patient and center level variables. Variables thought to 
be associated with a clinician’s decision to initiate adjunctive treatment with linezolid (vs. 
clindamycin) and with mortality will be selected for inclusion.  Overlap weighting on a 
propensity-score between the two groups will be performed with downstream adjustment for 
ICU stay, shock and debridement/source control. To take the clustered data structure into 
account, matching on center will be attempted if feasible, if not , propensity scores will be 
calculated using a mixed effects model with hospital as a random effect. Covariate balance 
between the two groups before and after generation of the weighted cohorts will be assessed by 
examining the standard mean difference for variables across the exposure categories. 
 
Power calculation 
Because no randomized clinical trials have previously compared adjunctive treatment options 
for GAS , the sample size estimation was derived from the largest retrospective study available 
3. In that study the overall in hospital mortality in adjunctive clindamycin group 6.5% in the 
propensity matched cohort. A published abstract, the only other published clinical data on the 
topic, found mortality between linezolid (n=26) and clindamycin (n=26) groups to be similar 
(clindamycin 11.5% and linezolid 7.7%)21 . This was confirmed by preliminary exploration of 
GAS blood stream infection cases  in our dataset (clindamycin 10%, linezolid 12%). 
 
Based on a mortality rate of 6.5% in the both groups as the most conservative estimate and a 
noninferiority margin of 5% with 1:4 allocation ratio, 1035 patients (828 clinda + 207 Linezolid) 
will be needed in total to achieve 80% power with a 2-sided α level of 0.05, allowing for 10% 
dropout. A 5% noninferiority margin was chosen as the maximal difference in mortality between 
treatments that would be clinically acceptable, by consultation with infectious disease, critical 
care specialists within the study team.  
 
Statistical analysis will be performed with R using Rstudio V2022.12.0.353 (R Foundation for 
Statistical Computing, Vienna, Austria). The reporting of the study will be in line with 
Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting 
guidelines20  
 
Sensitivity analysis  
The following sensitivity analysis will be performed: 
- 
Analysis of patients with proven GAS only (excluding those with probable GAS)  
- 
Analysis where hospital as a random effect is not included 
- 
An analysis which includes those who receive anti-toxin +/-3 days of index therapy to 
capture those treated empirically prior to culture results.  
- 
An analysis without PS weighting and only downstream logistical regression (as would 
be done in a classical observational study).  
- 
An analysis utilizing cloning methodology to account for potential immortal time bias 21 

 
Subgroup Analysis  
We will used similar methods as for the primary analysis to assess the impact of allocation to 
either treatment group on the following prespecified subgroups 
 
- 
Patients with NSTI or TSST  
- 
Patients with pressor dependent shock  
- 
Patients admitted to the ICU within one day (+/-) of index GA eligibility culture 
- 
Excluding patients with who received clindamycin that had a clindamycin non-
susceptible isolates (defined as resistant or intermediate or a positive D-test) 
- 
Excluding patients with who received clindamycin that had a clindamycin non-
susceptible isolates or had clindamycin antimicrobial susceptibility testing missing  
- 
Stratified by days of anti-toxin therapy (>1, >2, >3 days) 
 
Missing Data Plan 
Less than 5% missingness across key variables is expected 
 
 
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