Risk of preterm birth, small-for-gestational-age at birth, and stillbirth after receiving a booster dose of COVID-19 vaccine during pregnancy

Abstract

To assess risk of preterm birth, small-for-gestational-age (SGA) at birth, and stillbirth after first booster dose of COVID-19 vaccine (third dose) during pregnancy. We used provincial databases in Ontario to identify all live and stillbirths ≥20 weeks’ gestation (BORN birth registry), linked to COVID-19 vaccination data (vaccine registry). Individuals with an expected date of birth between December 15, 2021 (start of booster dose campaign) and August 31, 2022, who received primary series before pregnancy and became eligible for a booster while pregnant (≥6 months since second dose) were included. We estimated adjusted hazard ratios (aHR) and 95% confidence intervals (CI) for preterm birth (<37 weeks), SGA at birth (<10th percentile), and stillbirth, adjusted for calendar time, health behaviours, sociodemographic and pregnancy-related factors using propensity score methods. Among 31 751 births, 17 890 (56.3%) were born to individuals who had received a booster dose during pregnancy. The cumulative incidence of preterm birth was 7.4% among those who received a booster during pregnancy and 8.6% among those who did not. The risk of preterm birth was not associated with receiving a booster dose during pregnancy (aHR: 0.99, 95% CI 0.90–1.08). Similarly, there was no increased risk of SGA at birth among those receiving vs. not receiving a booster during pregnancy (8.5% vs. 9.8%; aHR: 0.87, 95% CI 0.80–0.95) or stillbirth (0.3% vs. 0.7%; aHR: 0.63, 95% CI 0.44−0.90). Receipt of a booster dose of COVID-19 vaccine during pregnancy was not associated with any increased risk of preterm birth, SGA at birth, or stillbirth.