Effect of Nirsevimab as a Primary Preventive Strategy on the Incidence, Severity, and Viral Etiology of Bronchiolitis: A Four‐Year Observational Study

Background: Bronchiolitis is a prevalent viral respiratory illness in infants, often caused by respiratory syncytial virus (RSV). It is the leading cause of hospitalization in children under 1 year of age. The introduction of Nirsevimab, a long-acting monoclonal antibody approved in 2022, aimed to reduce RSV-related hospitalizations and severe cases. This study assesses the impact of Nirsevimab on bronchiolitis incidence, severity, and viral etiology in Sicily from 2021 to 2025. Methods: This observational study included all pediatric patients hospitalized for bronchiolitis at the University of Catania's Pediatric Respiratory Unit between October 2021 and March 2025. Data were collected on viral etiology, hospital stay duration, clinical severity and respiratory support requirements across four epidemic seasons. Statistical analysis was used to compare outcomes across the different years, with a focus on the 2024–2025 season, which was the first to implement Nirsevimab immunization. Results: From 2022 to 2025, the incidence of hospitalized bronchiolitis decreased significantly, with a reduction of 84% in the 2024–2025 season compared to previous (2021–22, 2022–23, 2023–24) years (p < 0.0001). In the 2024–2025 season, a significant difference was observed in the percentage of RSV cases in 2025 compared to previous years and a significant reduction in the circulation of non-RSV viruses. The average hospital stay and clinical severity remained stable throughout the years. Only one hospitalized patient in the 2024–2025 season had received Nirsevimab, presenting with mild bronchiolitis. Conclusion: The introduction of Nirsevimab in Sicily led to a significant reduction in bronchiolitis incidence, with a reduction of RSV and non-RSV cases. Clinical severity and hospital stay duration remained unchanged. The findings support the efficacy of Nirsevimab in preventing severe bronchiolitis requiring hospitalization.