Effectiveness of maternal COVID-19 vaccination varies by gestational timing: results from a claims-based cohort study, 2020-2022

We evaluated COVID‑19 vaccine effectiveness (VE) during pregnancy by dose and trimester in a claims‑based cohort of 414,909 pregnant women from 25 December 2020 to 16 September 2022. Vaccination was modeled as a time‑varying exposure using weighted Cox proportional hazards models to estimate VE against medically-attended COVID‑19 and COVID‑19–related hospitalization. Overall, 58,975 (14.2%) women received ≥ 1 dose during pregnancy. We identified 39,202 COVID‑19 cases, including 1,130 hospitalizations, with most outcomes (> 93%) occurring during unvaccinated person‑time. VE against medically-attended COVID‑19 was low‑to‑moderate across trimesters, with the highest estimates after late second trimester vaccination (36.3%, 95% CI: 26.9–44.4) or ≥ 2 doses (40.7%, 95% CI: 26.3–52.3). VE against hospitalization was higher, albeit with wider confidence intervals (late second trimester, 73.1%, 95% CI: 13.3–91.6). COVID‑19 vaccination during pregnancy provided meaningful maternal protection, particularly against severe disease, with some indications that protection may be greatest when given during later gestational periods.