Real-world impact of nirsevimab immunisation and maternal RSV vaccination against respiratory disease on emergency department attendances and admissions: a multinational retrospective analysis

Background: Nirsevimab, a long-acting monoclonal antibody against respiratory syncytial virus (RSV), was recently introduced in Catalonia (Spain, 2023-2024 season onwards) and Italy (2024-2025 season). The United Kingdom (UK) instead introduced maternal RSV vaccination (RSVpreF) in the 2024-2025 season. Our aim was to analyse emergency department (ED) attendances and admissions to hospital following RSVpreF and nirsevimab introduction, with hospitals in Iceland and Romania, where no intervention was introduced, used as comparators. Methods: Multi-national retrospective analysis of ED attendances and admissions for all diagnoses, respiratory diagnoses excluding bronchiolitis, and bronchiolitis from all hospitals in Catalonia (Spain), four UK hospitals (Bristol, Edinburgh, Glasgow, and Leicester), and one hospital in Italy (Rome), Romania (Bucharest), and Iceland (Reykjavík) from April 2018 to March 2025. Bronchiolitis diagnoses in the 2024-2025 season were compared to previous pre-intervention seasons (2018-2023, excluding the 2020-2021 COVID-19 year) by applying a generalised linear model in Poisson regression to obtain risk ratios (RR) and 95% confidence intervals (95% CI). Findings: In the 2024-2025 season, in Catalonia, there was a reduction in the RR for bronchiolitis ED attendances and admissions in infants of age