Seasonal Trivalent Influenza Vaccination During Pregnancy and the Incidence of Stillbirth: Population-Based Retrospective Cohort Study

Background. Although antenatal influenza vaccination is an important public health intervention for preventing serious infection in pregnant women and newborns, reported vaccine coverage is often <50%. Concern for the safety to the fetus is a commonly cited reason for vaccine hesitancy and refusal. The incidence of stillbirth following pandemic vaccination has been previously studied; however, no population-based study has evaluated the incidence of stillbirth following seasonal trivalent influenza vaccination. Methods. We used probabilistic linking of perinatal and maternal vaccination records to establish a cohort of 58 008 births occurring between April 2012 and December 2013. Stillbirth was defined as birth >= 20 weeks' gestation with an Apgar score of zero at 1 and 5 minutes following delivery. Cox regression models adjusted for maternal smoking, Indigenous status, and propensity for vaccination were used to calculate adjusted hazard ratios (aHRs) in vaccinated and unvaccinated mothers. Results. A total of 5076 (8.8%) pregnant women received trivalent influenza vaccine and 377 stillbirths occurred. There were 5.0 and 3.0 stillbirths per 100 000 pregnancy-days among unvaccinated and vaccinated women, respectively. After adjustment, stillbirth was 51% less likely among vaccinated vs unvaccinated mothers (aHR, 0.49; 95% confidence interval [ CI], .29-.84). The largest relative reduction in stillbirths was observed for births occurring just after influenza season (aHR, 0.33; 95% CI, .12-.88). Conclusions. Mothers who received seasonal TIV during pregnancy were significantly less likely to experience stillbirth compared with unvaccinated mothers. These results support the safety of seasonal influenza immunization during pregnancy and suggest a protective effect.