The Path of Bronchiolitis Towards Intensive Care: Risk Factor Analysis in a Large Italian Cohort.

Bronchiolitis is the leading cause of hospitalization in infants under 12 months. While often self-limiting, a subset of cases evolves into severe disease requiring intensive care. This study aimed to identify risk factors for severe bronchiolitis in two consecutive respiratory syncytial virus (RSV) seasons (before and after the introduction of nirsevimab) in Southern Italy. A retrospective, multicenter cohort study was conducted on all infants ≤12 months hospitalized with bronchiolitis from October 2023 to March 2025. Patients were categorized by disease severity: those requiring Sub-Intensive or Intensive Care (IC group) and others (n-IC group). Demographic and clinical data, virological testing, and therapeutic interventions were analyzed. Multivariable logistic regression was used to identify independent risk factors for severe disease. Among 1056 hospitalized infants, 10.5% required intensive care. RSV was detected in 73.5% of tested patients and was significantly associated with severe outcomes. Independent risk factors for IC admission included younger age (<3 months), comorbidities, and RSV infection. A 33% reduction in bronchiolitis admissions was observed in the second season (post-nirsevimab), although the rate of severe cases remained stable (about 10% in both seasons). Younger age, comorbidities, and RSV infection are significant predictors of severe bronchiolitis. Although overall admissions decreased post-nirsevimab, severe cases persisted. These findings underscore the need for targeted preventive strategies and highlight the potential role of intermediate care approaches in managing bronchiolitis severity.