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

## TL;DR

This study found no increased risk of adverse birth outcomes in twin pregnancies following maternal influenza or pertussis vaccinations.

## Contribution

The study provides novel evidence on the safety of maternal influenza and pertussis vaccinations in twin pregnancies.

## Key findings

- No significant increased risk of preterm birth, stillbirth, or SGA infants was observed in vaccinated twin pregnancies.
- Pertussis vaccination was associated with a non-significant lower risk of SGA infants in Queensland and Northern Territory.

## Abstract

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

Multi‐jurisdictional data linkage cohort study.

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

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

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.

Adverse birth outcomes including preterm birth, stillbirth and SGA infants.

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).

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.