Impact of Nirsevimab on Risk Factors for Severe Bronchiolitis

Background: Bronchiolitis is the leading cause of hospitalization in infants under 12 months of age and can progress to severe disease requiring intensive care. Objectives: To identify risk factors for severe bronchiolitis over two consecutive respiratory syncytial virus (RSV) seasons—before and after the introduction of nirsevimab prophylaxis—in Southern Italy. Methods: We conducted a retrospective, multicenter cohort study including all infants aged ≤12 months hospitalized with bronchiolitis between October 2023 and March 2025. Demographic, clinical, and virological data, as well as therapeutic interventions, were collected. Multivariable logistic regression was used to identify independent risk factors for severe disease. Results: A total of 1,056 infants were included. Severe bronchiolitis accounted for approximately 10% of cases, with invasive mechanical ventilation needed in only 1%. Independent predictors of severe disease included younger age (aOR 2.10, 95% CI 1.15–3.85), comorbidities (aOR 3.85, 95% CI 1.75–8.45), RSV positivity (aOR 2.56, 95% CI 1.19–5.52) and chest radiographic consolidation (aOR 2.06, 95% CI 1.18–3.58). Procalcitonin >0.5 ng/mL approached statistical significance (aOR 2.07; 95% CI 0.98–4.37; p = 0.056). Although nirsevimab coverage was suboptimal in the Campania region during the 2024–2025 season due to organizational constraints, its introduction was associated with a reduction in overall hospital admissions and in the absolute number of severe cases. However, the proportion of severe cases did not differ significantly between pre- and post-nirsevimab seasons (10.9% versus S1: 9.9%, p 0.614). Following nirsevimab implementation, the risk profile for severe disease appeared to shift: age <3 months, historically the strongest predictor, was no longer significant. Conclusions: These findings suggest that nirsevimab may influence not only the incidence of bronchiolitis but also the factors associated with severe disease, with potential implications for risk stratification and hospital resource allocation.