A population-based test-negative matched case control analysis of SARS-CoV-2 vaccine effectiveness among pregnant people in Ontario, Canada

Objective — Pregnancy is a risk factor for severe SARS-CoV-2 infection, which can result in adverse pregnancy outcomes, thus making understanding vaccine effectiveness (VE) in this population important. This study aimed to assess the vaccine effectiveness of mRNA COVID-19 vaccines against symptomatic SARS-CoV-2 infection and COVID-19-related hospitalization in pregnant people.

Methods — Population-based matched test-negative case-control study of pregnant people aged 18–49 years, of 12 or more weeks’ gestation in Ontario, Canada, symptomatic with possible SARS-CoV-2 infection, and having at least one positive (n = 1842) or negative (n = 8524) real-time polymerase chain reaction (RT-PCR) SARS-CoV-2 test between December 14, 2020 and December 31, 2021. The exposure was receipt of ≥1 dose of mRNA COVID-19 vaccine versus no vaccination. Exposure was further stratified by number and recency of doses. The primary outcome was a positive SARS-CoV-2 RT-PCR test. As a secondary outcome, VE for COVID-19-related hospitalization was assessed.

Results — In the primary outcome analysis, there were 1821 positive cases, matched to 1821 negative controls. The mean (SD) maternal age was 31 (5) years. When compared to those unvaccinated, receipt of ≥1 dose was associated with an estimated VE of 39% (95% CI 29–48%) for symptomatic infection, and 85% (95% CI 72–92%) for COVID-19 hospitalization. Vaccine effectiveness estimates demonstrated waning with increased time since last vaccination.

Conclusions — MRNA COVID-19 vaccines provide protection against symptomatic COVID-19 illness and are highly effective at preventing severe illness in pregnant people. The observed effect of vaccine waning highlights the importance of booster doses to provide optimal protection for pregnant people.