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

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.

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.

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.

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.