Abstract
BACKGROUND/OBJECTIVES: Respiratory syncytial virus (RSV) bronchiolitis remains a leading cause of hospitalization in infants. In the 2024-2025 season, passive newborn immunization with nirsevimab, a long-acting anti-RSV monoclonal antibody, was introduced for the first time in Italy. However, the immunization campaign was not uniformly implemented on a regional basis due to supply and organizational difficulties. The aim of the study was to assess the real-world impact of nirsevimab prophylaxis during the 2024-2025 bronchiolitis season in four regions of Italy. METHODS: This multicenter observational study included infants <12 months hospitalized for bronchiolitis across four Italian centers. Hospitalizations due to RSV and non-RSV bronchiolitis were compared across the 2023-2024 and 2024-2025 seasons, in relation to the timing and coverage of nirsevimab's introduction in each of the four regions. RESULTS: Early and widespread nirsevimab administration was associated with a significant reduction in RSV hospitalizations and severity of disease. Centers located in regions that had delayed implementation of immunization observed higher RSV burden and intensive care unit admissions. Admissions for non-RSV bronchiolitis remained stable. CONCLUSIONS: Timely and universal administration of nirsevimab significantly reduced RSV hospitalizations and severity, while delayed implementation resulted in limited benefit. These findings support early and uniform prophylaxis to mitigate health disparities and seasonal pressure on pediatric healthcare systems.