Acute viral bronchiolitis before and after implementation of nirsevimab prophylaxis in Italy: a multicentre retrospective cohort study
Abstract
Summary Background Acute viral bronchiolitis is the leading cause of hospitalisation in the first year of life, with Respiratory Syncytial Virus (RSV) accounting for more than 60% of cases. A long-acting monoclonal antibody (nirsevimab) has been introduced for infants entering their first epidemic season. 2024–2025 season was the first with nirsevimab 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 Following implementation, emergency department visits decreased by 48% (5076 vs 2633), pediatric ward admissions by 48% (2734 vs 1473) and pediatric intensive care unit admissions by 61% (334 vs 131). Analyses adjusted for local rollout showed reductions of 82% (RR 0.18), 84% (RR 0.16) and 79% (RR 0.20), respectively. Post-implementation, infants were older (aOR 1.07, 95% CI 1.04–1.10) with lower odds for low- (aOR 0.67, 95% CI 0.55–0.82) and high-flow (aOR 0.8; 95% CI 0.65–0.99) oxygen-therapy. RSV remained predominant (934/1473, 63%), while non-RSV infections increased (334/2734, 12% vs 305/1473, 21%). Most infants had not received prophylaxis; 372 (25%) hospitalised despite nirsevimab 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 In Italy, nirsevimab 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.