Protective Effect of Maternal Influenza Vaccination on Influenza in Their Infants: A Prospective Cohort Study

BACKGROUND: Infants <6 months of age are too young to receive influenza vaccine, despite being at high risk for severe influenza-related complications. METHODS: To examine the effectiveness of maternal influenza vaccination in preventing influenza in their infants, we conducted a prospective cohort study of 3441 infants born at participating hospitals before the 2013–2014 influenza season. At the time of recruitment, their mothers completed a questionnaire about influenza vaccination status for the 2013–2014 season. A follow-up survey was conducted after the end of the 2013–2014 season to collect information regarding influenza diagnosis and hospitalization among infants. RESULTS: During the 2013–2014 influenza season, 71 infants (2%) had influenza diagnosed, and 13 infants (0.4%) were hospitalized with influenza. Maternal influenza vaccination (especially prenatal vaccination) decreased the odds of influenza among infants. The effectiveness of prenatal vaccination was 61% (95% confidence interval, 16%–81%), whereas that of postpartum vaccination was 53% (−28%–83%). Although maternal influenza vaccination was also associated with a decreased odds of influenza-related hospitalization among infants, vaccine effectiveness (73%) did not reach statistical significance, owing to the limited number of infants hospitalized because of influenza. CONCLUSIONS: The present findings indicated that pregnant women and postpartum women should receive influenza vaccination to protect their infants.