Analysis of mRNA vector efficacy

Analysis of mRNA vector efficacy


In an ongoing multinational, placebo-controlled, observer-blinded, pivotal efficacy trial, we randomly assigned persons 16 years of age or older in a 1:1 ratio to receive two doses, 21 days apart, of either placebo or the BNT162b2 vaccine candidate (30 μg per dose).

BNT162b2 is a lipid nanoparticle–formulated, nucleoside-modified RNA vaccine that encodes a prefusion stabilized, membrane-anchored SARS-CoV-2 full-length spike protein.

The primary end points were efficacy of the vaccine against laboratory-confirmed Covid-19 and safety.


A total of 43,548 participants underwent randomization, of whom 43,448 received injections: 21,720 with BNT162b2 and 21,728 with placebo.

There were 8 cases of Covid-19 with onset at least 7 days after the second dose among participants assigned to receive BNT162b2 and 162 cases among those assigned to placebo; BNT162b2 was 95% effective in preventing Covid-19 (95% credible interval, 90.3 to 97.6).

Similar vaccine efficacy (generally 90 to 100%) was observed across subgroups defined by age, sex, race, ethnicity, baseline body-mass index, and the presence of coexisting conditions.

Among 10 cases of severe Covid-19 with onset after the first dose, 9 occurred in placebo recipients and 1 in a BNT162b2 recipient

Absolute measure of effect (incidence rate difference): (8 cases/2,214 person-years) – (162 cases/2,222 person-years) = -0.07 or a rate of occurrence of 70 less #COVID19 cases per 1,000 person-years for vaccine vs. placebo

Relative measure of effect (incidence rate ratio): (8 cases/2,214 person-years) / (162 cases/2,222 person-years) = 0.05 or a 95% less rate of occurrence of #COVID19 cases for vaccine vs. placebo

If using rate: Reported 95% efficacy measure (calculated as 1 – incidence rate ratio) sounds like vaccine provides massive protection, but when you consider that the rate difference is only 70 less cases per 1,000 person-years for vaccine vs placebo, it’s really not much benefit

Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine
9 less cases per 1000