Obstetric and perinatal health outcomes after pertussis vaccination during pregnancy in Ontario, Canada: a retrospective cohort study

BACKGROUND: In February 2018, Canada’s National Advisory Committee on Immunization recommended maternal vaccination with tetanus–diphtheria–acellular pertussis (Tdap) vaccine during pregnancy to prevent severe pertussis infection in young infants. This study assessed the relation between maternal Tdap vaccination and obstetric and perinatal outcomes in Ontario. METHODS: We performed a population-based cohort study of all births from April 2012 to March 2017 using multiple linked health administrative databases. We used Cox regression with a time-dependent exposure variable to estimate adjusted hazard ratios (HRs) for preterm birth (< 37 wk), very preterm birth (< 32 wk) and stillbirth. We assessed remaining outcomes (gestational hypertension, chorioamnionitis, postpartum hemorrhage, severe postpartum hemorrhage, being small for gestational age, neonatal intensive care unit stay > 24 h, composite neonatal morbidity) using log-binomial regression to generate adjusted risk ratios (RRs). We adjusted estimates for potential confounding using propensity score weighting. RESULTS: Of 615 213 infants (live births and stillbirths), 11 519 were exposed to Tdap vaccination in utero. There was no increased risk for preterm birth (adjusted HR 0.98, 95% confidence interval [CI] 0.91–1.06), very preterm birth (adjusted HR 1.10, 95% CI 0.86–1.41), stillbirth (adjusted HR 1.15, 95% CI 0.82–1.60) or being small for gestational age (adjusted RR 0.96, 95% CI 0.90–1.02). The risks of a neonatal intensive care unit stay exceeding 24 hours (adjusted RR 0.82, 95% CI 0.76–0.88) and neonatal morbidity (adjusted RR 0.81, 95% CI 0.75–0.87) were decreased. There was no association with chorioamnionitis (adjusted RR 1.17, 95% CI 0.99–1.39), postpartum hemorrhage (adjusted RR 1.01, 95% CI 0.91–1.13) or severe postpartum hemorrhage (adjusted RR 0.79, 95% CI 0.55–1.13), but we observed a reduced risk of gestational hypertension (adjusted RR 0.87, 95% CI 0.78–0.96). INTERPRETATION: Our results complement evidence that maternal Tdap vaccination is not associated with adverse outcomes in mothers or infants. Ongoing evaluation in Canada is needed as maternal Tdap vaccination coverage increases in coming years.