The Impact of Nirsevimab on Bronchiolitis-Related Hospitalizations: A Multicenter Italian Retrospective Comparative Study

Introduction: Bronchiolitis is the leading cause of infant hospitalizations worldwide, primarily driven by respiratory syncytial virus (RSV). Methods: This multicenter retrospective comparative study assessed the impact of immunoprophylaxis with nirsevimab against RSV on bronchiolitis-related hospitalizations in Italy during the 2024-2025 winter season. Results: Data from nine Italian hospitals showed a substantial 48% decrease in bronchiolitis admissions compared to the previous season (438 vs. 832 admissions). Among hospitalized infants, only 23% had received immunoprophylaxis. RSV positivity dropped significantly among immunized patients (48%) versus non-immunized (73%, p < 0.0001), with fewer RSV-related coinfections. Discussion: While indicators of illness severity-such as ICU admission, respiratory support needs, and complications-were generally lower, no statistically significant differences in disease course were observed between immunized and non-immunized hospitalized infants. A shift in viral epidemiology was noted, with a reduction in RSV dominance and increased detection of rhinovirus and enterovirus, suggesting a pathogen replacement effect. The 2024-2025 season also saw a lower intubation rate (< 0.5%), pointing to an overall milder disease course. Conclusions: This study supports the effectiveness of nirsevimab in reducing RSV-associated hospitalizations and reshaping the virological landscape of bronchiolitis in Italy. Early and widespread implementation of immunoprophylaxis is recommended to maximize public health benefits.