Maternal influenza vaccine reduces the risk of RSV and related respiratory hospitalisations in early infancy: an epidemiological study.

Abstract

We examined the impact of maternal inactivated influenza vaccine (IIV) on respiratory syncytial virus (RSV)-hospitalisations in Western Australian infants aged <6 months. We conducted a population-based cohort study of births from April 2012 to December 2020 using linked, longitudinal data on births, deaths, perinatal data, hospitalisations, antenatal immunisation records and respiratory pathogen laboratory testing, with follow-up to 31st 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 hospitalisation in infants, adjusting for socio-demographic and perinatal factors and stratifying by vaccination trimester. Secondary outcomes included ICD-coded pneumonia, bronchiolitis and all-cause ARI-hospitalisations. The cohort comprised 288,059 infants. Overall, 87,277 mothers (30.3%) received a maternal IIV. There were 2,707 RSV-confirmed hospitalisations in 2,702 infants aged <6 months, the majority (69.7%) occurring in maternally unvaccinated infants. Exposure to maternal IIV was associated with an overall non-significant 10.0% reduction in RSV-hospitalisations (weighted and adjusted hazard ratio [aHR]:0.90, 95%CI:0.80-1.01) compared with maternally unvaccinated infants. However, by trimester, maternal IIV was most effective when administered in first (weighted aHR:0.65, 95%CI:0.50-0.85) or second (weighted aHR:0.75, 95%CI:0.62-0.90) trimesters. Maternal IIV was associated with a 35.0% and 25.0% reduced risk against RSV-hospitalisations when administered in the first or second trimester, respectively. Similar findings were observed for secondary outcomes. Non-specific effects should be included in the full public health evaluation of maternal IIV. These findings are important for policymakers and community to improve immunisation.