Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine

To the Editor: Polack et al. (Dec. 31) 1 report a vaccine efficacy of 94.8% against Covid-19 after two doses of the messenger RNA (mRNA) vaccine BNT162b2 (Pfizer–BioNTech). The authors also report a vaccine efficacy of 52.4% from after the first dose to before the second dose, but in their calculation, they included data that were collected during the first 2 weeks after the first dose, when immunity would have still been mounting. 1 We used documents submitted to the Food and Drug Administration 2 to derive the vaccine efficacy beginning from 2 weeks after the first dose to before the second dose . . .