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.