Effectiveness of the Pfizer-BioNTech COVID-19 Vaccine in Pregnant Women


JOURNAL WATCH – New England Journal of Medicine

Hana M. El Sahly, MD, reviewing

In an observational study of pregnant women in Israel, the BNT162b2 vaccine was 78% effective at preventing symptomatic and asymptomatic SARS-CoV-2 infections.

Pregnant and postpartum women are at excess risk for complications of SARS-CoV-2 infection, yet they were excluded from the Phase 3 clinical trials testing COVID-19 vaccine efficacy (although they are among those recommended via Emergency Use Authorization to receive the vaccines). Researchers in Israel evaluated the effectiveness of the Pfizer-BioNTech mRNA vaccine (BNT162b2) in a retrospective cohort of all pregnant women from a healthcare system representing 27% of the Israeli population. A total of 7530 women who were vaccinated were matched by calendar day to 7530 women who were not vaccinated; the primary outcome was SARS-CoV-2 infection 28 days after the first dose.

Beginning 28 days after the first dose (mean follow-up, 37 days), 10 infections occurred in the vaccinated group and 46 in the unvaccinated group (vaccine efficacy, 78%). After the first dose, the adjusted hazard ratio for SARS-CoV-2 infection was no different between groups within 10 days (aHR, 0.96) but was statistically significantly reduced at 11–27 days (aHR, 0.46) and ≥28 days (aHR, 0.22).


It’s important to note that the cases in this study included asymptomatic and symptomatic infections, which partially explains the somewhat lower effectiveness that was observed. Important limitations are the lack of safety data and the short follow-up and observational design. Still, taken together with the large safety database from the CDC, these findings support the effort to continue prioritizing vaccination of pregnant women.

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