The Risk of Adverse Birth Outcomes Among Twin Pregnancies After Influenza and Pertussis Vaccinations During Pregnancy: A Data Linkage Study.

OBJECTIVE: To compare risks of adverse birth outcomes between maternally vaccinated and unvaccinated twin pregnancies.

DESIGN: Multi-jurisdictional data linkage cohort study.

SETTING: All registered births in Queensland (Qld) and Northern Territory (NT), Australia between 1 January 2012 and 31 December 2017.

POPULATION: Twin pregnancies with infants born ≥ 20 weeks gestation and weighing ≥ 400 g.

METHODS: We used Cox proportional-hazard models to calculate risk, with maternal vaccination status as the time-varying exposure, and national birthweight percentile charts specific for Australian-born twins to accurately reflect risk among small for gestational age (SGA) infants.

MAIN OUTCOME MEASURES: Adverse birth outcomes including preterm birth, stillbirth and SGA infants.

RESULTS: Among our cohort of n = 11 435 infants, there was no statistically significant increased risk of preterm births, stillbirths or SGA infants between women who received a maternal influenza or pertussis vaccination and unvaccinated women. We observed a non-significant lower risk of SGA infants among pertussis vaccinated Qld (aHR 0.85, 95% CI 0.70-1.03) and NT women (aHR 0.50, 95% CI 0.17-1.44), and a lower risk of preterm infants among influenza vaccinated Qld women (aHR 0.93, 95% CI 0.84-1.03) and pertussis vaccinated NT women (aHR 0.78, 95% CI 0.44-1.38).

CONCLUSION: Maternal influenza and pertussis vaccinations did not increase the risk of preterm birth, stillbirth or SGA infants among twin pregnancies. These novel and important findings alleviate safety concerns for a cohort that carries a higher baseline risk of adverse birth outcomes compared to singleton pregnancies.