Effectiveness of Maternal Influenza Vaccination on Influenza and Acute Respiratory Infection in Infants

BACKGROUND: Influenza vaccination is recommended for pregnant women, but evidence quantifying effectiveness in the prevention of influenza and acute lower respiratory infection (ALRI) in infants in the first 6 months of life is limited. METHODS: We conducted a population-based, matched cohort study using New South Wales Perinatal Data from 2016 to 2019 linked to hospitalization and disease notifications. Annual incidence of influenza notification/hospitalization and all-cause ALRI hospitalization among infants during the first 6 months of life was estimated. Vaccine effectiveness (VE) was estimated from adjusted hazard ratios (HR) calculated using Cox models. RESULTS: During follow-up of 147,907 matched infant pairs, there were 1041 influenza notifications/hospitalizations and 7496 ALRI hospitalizations. Influenza notification/hospitalization rates per 1000 person-years ranged between 7.40 in 2016 to 13.85 in 2019 in the unvaccinated group and from 5.82 in 2016 to 8.47 in 2019 in the vaccinated group. Maternal influenza VE [95% confidence interval (CI)] against influenza notification/hospitalization in the first 6 months of life varied from nonsignificant in 2016, 2017 and 2018 [20% (-17% to 45%), 12% (-11% to 31%), 23% (-17% to 50%), respectively] to 28% (12%-41%) in 2019; the pooled VE over 4 years was 21% [10%-31%]. No significant effect on all-cause ALRI hospitalization was observed. CONCLUSION: While the protective effect of maternal influenza vaccine in infants was modest, influenza hospitalization rates are highest in the first 6 months of life, and there is currently no vaccine available for this age group, underscoring the benefit of maternal vaccine for pregnant women and their infants.