Question explored with the scientific record
In the Pfizer C4591001 phase 3 trial, what was BNT162b2 efficacy against confirmed COVID-19 and the absolute risk difference between vaccine and placebo arms?
The 95% efficacy figure hides that the absolute risk reduction was only about 0.7 percentage points.
The Pfizer C4591001 trial reported 8 COVID-19 cases among 21,720 vaccinated participants versus 162 cases among 21,726 in the placebo group [2]. That works out to a relative risk reduction of about 95%, but an absolute risk reduction of just 0.7% [2]. In plain terms, about 143 people needed to be vaccinated to prevent one confirmed case during the trial period.
| Measure | Vaccine arm | Placebo arm |
|---|---|---|
| Confirmed COVID-19 cases | 8 of 21,720 | 162 of 21,726 |
| Absolute risk of infection | 0.04% | 0.75% |
| Absolute risk reduction | 0.7 percentage points | - |
| Relative risk reduction | ~95% | - |
The trial was funded by Pfizer/BioNTech, and the primary endpoint was PCR-confirmed symptomatic infection, not hospitalization or death [1]. The follow-up was only about 2.4 months [1]. The relative risk reduction is what the headlines quoted; the absolute reduction is what a healthy person actually gained.
My call: the vaccine did reduce confirmed infections in the trial, but the benefit was far smaller in absolute terms than the 95% figure suggests. Confidence: high on the numbers, moderate on whether that benefit justifies the intervention for any given individual.
Sources used 2
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Efficacy and Safety of AZD1222, BNT162b2 and mRNA-1273 vaccines against SARS-CoV-2
Critical review finds Moderna and Pfizer vaccines show high efficacy and safety in younger, healthier populations, while AstraZeneca's AZD1222 shows lower efficacy and notable trial biases, highlighting limited external validity and the need for broader, peer-reviewed data and v…
DOI: 10.32391/ajtes.v5i1.178 -
Outcome Reporting Bias in COVID-19 mRNA Vaccine Clinical Trials
A critical epidemiologic appraisal of Pfizer/BioNTech and Moderna COVID-19 mRNA vaccine phase III trials demonstrates that the reported relative risk reductions vastly exceed the reported absolute risk reductions, indicating outcome reporting bias in publicly available efficacy …
DOI: 10.3390/medicina57030199