The role of vaccination in maternal and perinatal outcomes associated with COVID-19 in pregnancy

Importance: This study was established to demonstrate the impact of COVID-19 in pregnancy and determine the impact of vaccination on pregnant women and their infants. Objective: To investigate the impact of vaccination on maternal and perinatal outcomes associated with SARS-CoV-2 infection in pregnancy. Design, Setting, and Participants: Population-level surveillance of SARS-CoV-2-infected pregnant women and gender diverse pregnant people and their infants (CANCOVID-Preg) between April 5, 2021 (beginning of 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 into 2023 for pregnancy conclusion and infant outcome. 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 30-35 (46.3%) and among those of White race (55.9%). 72% (n=14,367) of cases were vaccinated and 28% (n=5,532) 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=2,942) were vaccinated during pregnancy. Cases occurred during both Delta (n=6,120) and Omicron (n=13,799) variant time periods. Vaccination was associated with lower risk of hospitalization (Delta: risk ratio [RR]=0.38 [0.30-0.48], absolute risk difference [ARD]=8.7 [7.3-1.2]; Omicron: RR=0.38 [0.27-0.53], ARD=3.8 [2.4-5.2]), critical care unit admission (Delta: RR= 0.10 [0.04-0.26], ARD=2.4 [1.8-2.9]; Omicron: RR=0.10 [0.03-0.29], ARD=0.85 [0.27-1.44]), and preterm birth (Delta: RR=0.80 [0.66-0.98], ARD=1.8 [0.3-3.4]; Omicron: RR=0.64 [0.52-0.77], ARD=4.1 [2.0-6.2]). In multivariable analyses, vaccination was still associated with lower hospitalization risk in both variant time periods, after controlling for co-morbid conditions. In Omicron, compared to the vaccinated group, those unvaccinated had an adjusted RR of hospitalization of 2.43 (1.72-3.43). In Delta, those unvaccinated had an adjusted RR of hospitalization of 3.82 (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.