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.

ObjectivesUnderstanding 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. DesignA surveillance time series study analysing routinely collected RSV notification data. SettingsNational-level weekly RSV notifications collected by the Health Service Executive (HSE)-Health Protection Surveillance Centre (HPSC) in Ireland from 2012 to 2024. ParticipantsInfants (<1 year), young children (1-4 years), and older adults ([&ge;]65 years) with laboratory-confirmed RSV, from within the corresponding Irish population. Outcome measuresAnnual 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. ResultsThe study examined the increasing incidence of RSV post-COVID 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 two months earlier than in pre-COVID pandemic seasons (p < 0.01). Cross-correlation factor (CCF) 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 3-5 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 (p<0.001) and peak intensities increasing by over 60% from 2021/22 to 2023/24. ConclusionThis 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. Data Availability StatementAll data relevant to the study are included in the article or uploaded as supplementary information. Weekly RSV notification data reported by the HSE-Health Protection Surveillance Centre (HPSC) from week 40, 2012 to week 20, 2024 is available at https://respiratoryvirus.hpsc.ie/ Strengths and limitations of this studyO_LIA national, prospectively collected weekly surveillance dataset spanning twelve seasons (pre- and post-COVID-19) provided a robust basis for analysing RSV trends in Ireland. C_LIO_LIAge-stratified analysis (<1 year, 1-4 years, [&ge;]65 years) allowed comparison of temporal patterns across key population groups. C_LIO_LIApplication of time-lag correlation methods quantified inter-age transmission lags, supporting community-level planning and preparedness. C_LIO_LIThe study establishes a national pre-immunisation epidemiological baseline to inform assessment of future RSV immunisation programmes. C_LIO_LIThe absence of national data on RSV-related general practitioner (GP) visits, hospitalisations, or critical-care demand limited evaluation of the full clinical burden. C_LI