| Summary: | Introduction:: Respiratory Syncytial Virus (RSV) is a single-stranded RNA virus and a major cause of hospitalisations in paediatric and geriatric populations. In the Northern Hemisphere, the RSV season is typically between October and March. Following the introduction of Non-pharmaceutical Interventions (NPIs), in response to the COVID-19 pandemic, disruptions in seasonality have been observed. Methods:: We used an age-structured, deterministic SE2I2R model with time-dependent contact rates to study RSV hospitalisations and seasonality in the context of specific NPIs in Wales. The transmission process was linked to a clinical events model, to allow comparison to paediatric admissions data from Public Health Wales. The model was calibrated using Welsh demographics, social contact surveys and a severity index of Welsh NPI impact. Results:: Admissions data revealed three out-of-season outbreaks (Autumn 2020, Autumn 2021 and Summer 2022). A surge of admissions in Winter 2022-23 and Winter 2023-24 were forecasted, with peak timings correctly predicted, despite a more protracted outbreak observed in the data. Approximately, 90% of RSV admissions in Wales from 2016-22 were in infants under 1 year old; with the greatest shift in admissions age-structure in 2-4 year olds (quintupling in 2021). The model predicted a rapid return to pre-pandemic patterns after disruptions. Discussion/Conclusions:: Out-of-season peaks chiefly coincided with NPI relaxation. The post-pandemic response of RSV, in terms of timings, magnitude and age-structure shift, were all broadly consistent with simple interruptions in population exposure during the pandemic and the build up of immune naïve cohorts. Our model forms the basis of medium-term projections for paediatric RSV admissions in Wales.
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