Virus Detection and Hospitalization Rate in Infants With Bronchiolitis After Nirsevimab Introduction: A Retrospective Comparative Two‐Season Study

INTRODUCTION: Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections (LRTIs) in infants. In 2024, the monoclonal antibody nirsevimab became available in Italy for all newborns during their first RSV season. AIMS: The aims were to assess whether the introduction of nirsevimab was associated with changes in respiratory virus circulation and hospitalization rates among children admitted with bronchiolitis. METHODS: A retrospective study was conducted, comparing two epidemic seasons: pre-nirsevimab (2023-2024) and post-nirsevimab (2024-2025). All infants under 12 months hospitalized for bronchiolitis were included. Viral detection was performed using multiplex PCR on a nasopharyngeal swab. Emergency department visits, pediatric ward, and PICU admissions were analyzed. RESULTS: A total of 168 patients with bronchiolitis were enrolled across the two epidemic seasons, with 119 cases in the pre-nirsevimab season and 49 in the post-nirsevimab season. Regarding virus circulation, influenza virus cases increased from 5/119 (4.2%) in the pre-nirsevimab season to 7/48 (14.6%) in the post-nirsevimab season. Bocavirus was not detected during the pre-nirsevimab season (0/119) but was identified in 4/48 cases (8.3%) in the post-nirsevimab season. RSV-related hospitalizations decreased in the post-nirsevimab season, corresponding to a 59% reduction (95% CI 0.30-0.57; p < 0.001) in pediatric ward admissions. Similarly, the number of ED visits and PICU admissions for bronchiolitis decreased substantially, with reductions of 52% (95% CI 0.39-0.59; p < 0.001) and 60% (95% CI 0.17-0.99; p < 0.05), respectively. Hospitalized infants were significantly older, with a marked reduction in admissions among those ≤ 3 months of age. CONCLUSION: Nirsevimab has proven effective in reducing severe RSV-related bronchiolitis, significantly easing the burden on healthcare systems during RSV epidemics.