Heterogeneous Reduction in Paediatric RSV Hospitalisations Following Regional Nirsevimab Immunisation Strategies: A Multicentre Study

Objectives: To describe paediatric RSV-hospitalisation trends over six seasons and changes following regional nirsevimab strategies in Italy. Methods: A Retrospective study across 19 Italian paediatric units was conducted analysing RSV-related hospitalisations in patients aged <18 years from January 2019 to March 2025. Centres were grouped by regional nirsevimab eligibility criteria (Groups A-G). Group-level ratios of 2024-2025 to pre-immunisation hospitalisations were estimated with a negative-binomial model. Influenza was analysed as a negative control RESULTS: 10,915 RSV hospitalisations were recorded. In 2024-2025 RSV season, hospitalisations decreased 43.6% overall. The largest reductions were in groups with broader eligibility and earlier initiation (Group B: ratio 0.28, 95% CI 0.22-0.36; Group A: 0.40, 95% CI 0.22-0.74) and Group F (0.24; single centre, interval indicative). Groups C, D, and E showed more modest reductions (ratios 0.58-0.70), with Group C spanning unity (0.58, 95% CI 0.27-1.25). Group G showed no clear change once baseline instability was accounted for (1.84, 95% CI 1.15-2.94). Conclusions: These population-level study describe a temporal association between the 2024-2025 nirsevimab rollout and reduced RSV hospitalisations, particularly with broader eligibility and timely rollout. The study design cannot establish causation or compare the effectiveness of regional strategies.