The Role of Vaccination in Maternal and Perinatal Outcomes Associated With COVID-19 in Pregnancy.

Importance: Gaps in knowledge exist about the impact of COVID-19 and vaccination on pregnancy outcomes. Objective: To investigate the impact of vaccination on maternal and perinatal outcomes associated with SARS-CoV-2 infection in pregnancy. Design, Setting, and Population: Population-level surveillance of pregnant individuals infected with SARS-CoV-2 and their infants using the CANCOVID-Preg database between April 5, 2021 (beginning of the Delta variant time period and initiation of recommendations for vaccination in pregnancy in Canada), and December 31, 2022. Cases were identified based on COVID-19 diagnoses in pregnancy in 9 of 13 Canadian provinces/territories. Cases occurring through 2022 were followed up into 2023 for pregnancy conclusion and infant outcomes. Exposure: SARS-CoV-2 infection in pregnancy, with or without prior vaccination. Main Outcomes and Measures: COVID-19-associated hospitalization, critical care unit admission, and preterm birth. Results: Of 26 584 cases identified, 19 899 cases were eligible for analysis. Among these, most infections occurred among those aged 30 to 35 years (46.3%) and among those of White race (55.9%). A total of 72% (n = 14 367) of cases were vaccinated and 28% (n = 5532) were unvaccinated prior to their COVID-19 diagnosis. Among those vaccinated prior to COVID-19 diagnosis, 80% (n = 11 425) were vaccinated prior to pregnancy and 20% (n = 2942) were vaccinated during pregnancy. Cases occurred during both Delta (n = 6120) and Omicron (n = 13 799) variant time periods. Vaccination was associated with lower risk of hospitalization (Delta: relative risk [RR], 0.38 [95% CI, 0.30-0.48]; absolute risk difference [ARD], 8.7% [95% CI, 7.3%-10.2%]; Omicron: RR, 0.38 [95% CI, 0.27-0.53]; ARD, 3.8% [95% CI, 2.4%-5.2%]), critical care unit admission (Delta: RR, 0.10 [95% CI, 0.04-0.26]; ARD, 2.4% [95% CI, 1.8%-2.9%]; Omicron: RR, 0.10 [95% CI, 0.03-0.29]; ARD, 0.85% [95% CI, 0.27%-1.44%]), and preterm birth (Delta: RR, 0.80 [95% CI, 0.66-0.98]; ARD, 1.8% [95% CI, 0.3%-3.4%]; Omicron: RR, 0.64 [95% CI, 0.52-0.77]; ARD, 4.1% [95% CI, 2.0%-6.2%]). In multivariable analyses, vaccination was still associated with lower hospitalization risk in both variant time periods after controlling for comorbid conditions. In Omicron, compared with the vaccinated group, those unvaccinated had an adjusted RR of hospitalization of 2.43 (95% CI, 1.72-3.43). In Delta, those unvaccinated had an adjusted RR of hospitalization of 3.82 (95% CI, 2.38-6.14). Conclusions and Relevance: Vaccination against SARS-CoV-2 prior to and during pregnancy, before COVID-19 diagnosis, was associated with a lower risk of severe maternal disease and preterm birth regardless of variant time period.