Maternal Influenza Vaccine Reduces the Risk of Respiratory Syncytial Virus and Related Respiratory Hospitalizations in Early Infancy: An Epidemiological Study

BACKGROUND: We examined the impact of maternal inactivated influenza vaccine (IIV) on respiratory syncytial virus (RSV) hospitalizations in Western Australian infants aged ≤6 months. METHODS: We conducted a population-based cohort study of births from April 2012 to December 2020 using linked, longitudinal data on births, deaths, perinatal data, hospitalizations, antenatal immunization records, and respiratory pathogen laboratory testing, with follow-up to 31 December 2021. We performed Cox proportional hazards models with inverse probability treatment weighting. Mother's receipt of IIV was the exposure against the primary outcome, RSV-confirmed hospitalization in infants, adjusting for sociodemographic and perinatal factors and stratifying by vaccination trimester. Secondary outcomes included International Classification of Diseases-coded pneumonia, bronchiolitis, and all-cause acute respiratory infection hospitalizations. RESULTS: The cohort comprised 288 059 infants. Overall, 87 277 mothers (30.3%) received a maternal IIV. There were 2707 RSV-confirmed hospitalizations in 2702 infants aged ≤6 months, the majority (69.7%) occurring in maternally unvaccinated infants. Exposure to maternal IIV was associated with an overall nonsignificant 10.0% reduction in RSV hospitalizations (weighted adjusted hazard ratio [aHR], 0.90 [95% confidence interval {CI}, .80-1.01]) compared with maternally unvaccinated infants. However, by trimester, maternal IIV was most effective when administered in first (weighted aHR, 0.65 [95% CI, .50-.85]) or second (weighted aHR, 0.75 [95% CI, .62-.90]) trimesters. Maternal IIV was associated with a 35.0% and 25.0% reduced risk against RSV hospitalizations when administered in the first or second trimester, respectively. Similar findings were observed for secondary outcomes. CONCLUSIONS: Nonspecific effects should be included in the full public health evaluation of maternal IIV. These findings are important for policymakers and community to improve immunization.