The Impact of Maternal Influenza Infection and Vaccination During Pregnancy on Birth Outcomes

BACKGROUND: Pregnant women remain vulnerable to seasonal influenza and its complications, and both maternal infection and vaccination during pregnancy may influence birth outcomes. This study aimed to examine these associations and the role of the timing of infection or vaccination. METHODS: This retrospective cohort study used electronic health records of 42,689 mother-infant pairs from 2012 to 2019 in Pennsylvania. Cox regression with time-varying exposure was performed to examine the associations between maternal influenza infection and vaccination and birth outcomes. RESULTS: Influenza infection during pregnancy was associated with an increased hazard of adverse birth outcomes, including cesarean delivery (adjusted hazard ratio [aHR]: 1.31; 95% confidence interval [CI]: 1.00-1.57), low birthweight (aHR: 1.40; 95% CI: 1.05-1.87), very low birthweight (aHR: 2.28; 95% CI: 1.22-4.27), preterm birth (aHR: 1.34; 95% CI: 1.01-1.77) and extreme preterm birth (aHR: 1.95; 95% CI: 1.01-3.77), with strongest effects in the first trimester. In contrast, maternal influenza vaccination was associated with significantly reduced hazards of low birthweight (aHR: 0.84, 95% CI: 0.78-0.89), very low birthweight (aHR: 0.67, 95% CI: 0.57-0.80), preterm birth (aHR: 0.83, 95% CI: 0.79-0.89) and extreme preterm birth (aHR: 0.59, 95% CI: 0.50-0.70). CONCLUSIONS: Early pregnancy influenza infection was associated with increased risks of adverse birth outcomes, while vaccination reduced the risks of low birth weight and preterm birth. During the influenza vaccination season, women should be encouraged to get influenza vaccine as early in pregnancy as possible for their own protection and for their infant's health.