Respiratory syncytial virus (RSV) notifications and trends in the transmission cycles from infants and young children to older adults in Ireland: an analysis of incidence shifts over a decade

OBJECTIVES: Understanding the epidemiological shifts of respiratory syncytial virus (RSV) is essential to inform public health interventions, particularly given its increased burden on healthcare systems post-COVID-19 pandemic. This study aimed to examine age-specific trends and seasonal variations in RSV incidence, considering the recent introduction of a newborn RSV immunisation programme in Ireland. DESIGN: A surveillance time series study analysing routinely collected RSV notification data. SETTINGS: National-level weekly RSV notifications collected by the Health Service Executive-Health Protection Surveillance Centre in Ireland from 2012 to 2024. PARTICIPANTS: Infants (<1 year), young children (1-4 years) and older adults (≥65 years) with laboratory-confirmed RSV, from within the corresponding Irish population. OUTCOME MEASURES: Annual trends in RSV epidemiology with special reference to the pre- and post-COVID-19 winter surges, and the time lag in age-related transmission to peak incidence among the various age groups. Data were analysed to evaluate incidence rates, peak timing, age-related transmission trends and lag times before and after the COVID-19 pandemic. RESULTS: The study examined the increasing incidence of RSV post-COVID-19 and a significant shift toward earlier RSV peaks in recent years (2021/2022, 2022/2023 and 2023/2024 seasons) in Ireland, with the onset and peak of the season nearly 2 months earlier than in pre-COVID-19 pandemic seasons (p<0.01). Cross-correlation factor analysis indicated a sequential spread of RSV infections, where a peak in older adults followed an initial rise in cases among infants and young children, within a 3- to 5-week period (maximum cross-correlation=0.86 at lag 4 weeks, p<0.001). Post-pandemic, infants exhibited higher infection rates, with incident rates significantly higher in all seasons post-COVID-19 (p<0.001) and peak intensities increasing by over 60% from 2021/2022 to 2023/2024. CONCLUSION: This analysis highlights an early seasonal onset and intensified RSV burden among infants in recent winters (2021/2022, 2022/2023 and 2023/2024 seasons). Quantifying the time lag for the community-level RSV transmission from infants and young children to older adults will offer insights to optimise RSV intervention strategies as a 'life-course approach' to alleviate healthcare system pressures during peak seasons.