BNT162b2 mRNA COVID-19 vaccine effectiveness in pregnancy: Emulating trial NCT04754594 using observational data from Norwegian health registries

Nov 7, 2025Vaccine

Effectiveness of the Pfizer COVID-19 vaccine during pregnancy using Norwegian health data

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Abstract

Among 5790 pregnancies, vaccination was associated with a 20% lower risk of SARS-CoV-2 infection.

The intention-to-treat analysis showed a 20% reduction in infection risk for vaccinated individuals compared to unvaccinated individuals.
The as-treated analysis indicated a 39% lower risk of infection among those who received both doses of the vaccine.
The findings from this analysis were comparable to results from the randomized trial NCT04754594, which reported a 41% risk reduction.
SARS-CoV-2-related hospitalizations could not be assessed due to their rarity in the population studied.
This trial emulation suggests that observational data may be useful for assessing vaccine effectiveness in pregnant populations.

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Previous observational studies of mRNA COVID-19 vaccine effectiveness in pregnancy differ from the randomized trial NCT04754594, limiting causal inference. Using nationwide Norwegian health registry data, we emulated trial NCT04754594 to assess agreement of point estimates for BNT162b2 vaccine effectiveness in pregnancy. We included pregnancies reaching gestational week 24 after January 1, 2021, and delivering before November 30, 2022. Vaccinated individuals received a first dose between weeks 24-34 of gestation, with a second dose assumed within 21 days (intention-to-treat, ITT); the as-treated (AT) analysis required second-dose administration. We sequentially matched vaccinated individuals by gestational week to those who remained unvaccinated until the same gestational week, using propensity scores based on baseline covariates and last menstrual period date. Outcomes were SARS-CoV-2 infection and SARS-CoV-2-related hospitalization until one month post-delivery. Among 5790 pregnancies (2895 first dose-vaccinated, 2895 unvaccinated), mean maternal ages were 31.1 years (SD 4.3) in the unvaccinated, and 31.4 years (SD 4.6) vaccinated group. Mean BMI was 24.7 (SD 4.6) and 24.9 (SD 5.0), respectively. The ITT analysis showed that vaccination was associated with a 20 % lower risk of infection (IRR: 0.80, 95 % CI: 0.48, 1.13); the AT analysis showed a 39 % lower risk (IRR: 0.61, 95 % CI: 0.27, 0.95), comparable to that of the trial (41 % risk reduction). SARS-CoV-2-related hospitalizations could not be evaluated due to the rarity of the outcome in our population. This trial emulation demonstrated satisfactory statistical alignment with trial results, supporting the use of observational data to assess vaccine effectiveness in pregnant populations.

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BNT162b2 mRNA COVID-19 vaccine effectiveness in pregnancy: Emulating trial NCT04754594 using observational data from Norwegian health registries

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