Effectiveness of maternal influenza vaccine to infants: A population-based cohort study

BACKGROUND: Infants younger than 6 months are at increased risk of severe influenza-related complications but they are not eligible for vaccination. This study aimed to evaluate the effectiveness of maternal influenza vaccination in preventing influenza infection and influenza-associated hospitalization in infants. METHODS: We conducted a nationwide, population-based cohort study using linked data from Taiwan's National Health Insurance Research Database and the Maternal and Child Health Database. A total of 1,928,287 singleton infants born to primiparous mothers between 2004 and 2019 were included. Maternal influenza vaccination was identified using treatment codes. Outcomes were physician-coded influenza infection and influenza-related hospitalization within the first year of life. Vaccine effectiveness (VE) was calculated as (1 - adjusted odds ratio) × 100%, using multivariable logistic regression. RESULTS: Maternal vaccination was associated with a 7.4% reduction in influenza infection (adjusted odds ratio aOR: 0.93; 95% CI: 0.87-0.98) and a 21.4% reduction in hospitalization (aOR: 0.79; 95% CI: 0.65-0.95). Among infants aged 0-3 months, VE against infection was 26.4%; among those aged 4-6 months, VE against hospitalization was 33.1%. Vaccination at ≥28 weeks of gestation conferred greater protection than earlier vaccination. VE was significant in term infants but not in preterm infants, which was likely due to limited case numbers. CONCLUSIONS: Maternal influenza vaccination was associated with reduced risk of influenza infection and hospitalization in infants under 6 months, especially when administered in the third trimester. These findings support current recommendations to vaccinate pregnant women against influenza.