Acute viral bronchiolitis before and after implementation of nirsevimab prophylaxis in Italy: a multicentre retrospective cohort study

Background: availability in Italy. We assessed its impact on bronchiolitis-related utilisation and clinical and virological characteristics of hospitalised infants. Methods: We conducted a multicentre retrospective cohort study including infants <12 months with all-cause bronchiolitis during two seasons: pre-implementation (October 2023-April 2024) and post-implementation (October 2024-April 2025). Data were collected from 30 university hospitals across 15 regions using ICD-10 codes. Outcomes were compared using propensity score matching and multivariate regression analyses. Findings: were younger (5.2 ± 3.5 vs 2.8 ± 1.9, p < 0.001) and had more comorbidities (11%, 82/745 vs 22%, 37/168, p < 0.001). Interpretation: was associated with reduced all-cause bronchiolitis-related utilisation and milder disease. Underlying conditions remained a determinant for severe outcomes despite prophylaxis. Funding: No funding was received for this study.