For a study, researchers sought to investigate the risk of poor pregnancy outcomes following SARS-CoV-2 immunization during pregnancy. The registry-based retrospective cohort analysis comprised 157,521 singleton pregnancies that ended after 22 gestational weeks between January 1, 2021 and January 12, 2022 or January 15, 2022. The Pregnancy Register and the Medical Birth Registry were connected to vaccination and other registries to identify exposure and background variables. National vaccination registries were used to collect information on two mRNA vaccines, BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna), as well as one viral vector vaccine, AZD1222 (AstraZeneca).

Cox regression models were used to assess the risk of preterm birth and stillbirth, with gestational day as the time measure and vaccination as a time-dependent exposure variable. Logistic regression was used to assess the risk of small for gestational age, poor Apgar score, and neonatal care admission. To aggregate data across nations, random-effects meta-analysis was employed.

The mean maternal age at the time of delivery was 31 years among the 157,521 singleton births included in the study (103,409 in city and 54,112 in other), and 28,506 (18%) were vaccinated against SARS-CoV-2 (12.9% with BNT162b2, 4.8% with mRNA-1273, and 0.3% with AZD1222) while pregnant. During the first, second, and third trimesters, 0.7%, 8.3%, and 9.1% of persons delivering were vaccinated, respectively. Preterm birth (6.2 vs 4.9 per 10,000 pregnancy days; adjusted hazard ratio [aHR], 0.98 [95% CI, 0.91 to 1.05]; I^2=0%; P for heterogeneity=.60]), stillbirth (2.1 vs 2.4 per 100,000 pregnancy days; aHR, 0.86 [95% CI, 0.63 to 1.17]), and small for gestational age (7.8 vs 8.5% ; difference, –0.6% [95% CI, –1.3% to 0.2%]; adjusted OR [aOR], 0.97 [95% CI, 0.90 to 1.04]), low Apgar score (1.5% vs 1.6%; difference, –0.05% [95% CI, –0.3% to 0.1%]; aOR, 0.97 [95% CI, 0.87 to 1.08]), or neonatal care admission (8.5% vs 8.5%; difference, 0.003% [95% CI, –0.9% to 0.9%]; aOR, 0.97 [95% CI, 0.86 to 1.10]). 

Vaccination against SARS-CoV-2 during pregnancy was not substantially related with an increased risk of unfavorable pregnancy outcomes in the population-based investigation. The bulk of the immunizations were given using mRNA vaccines during the second and third trimesters of pregnancy, which should be taken into account when interpreting the results.