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During the 2023-2024 season, nirsevimab significantly reduced the risk of bronchiolitis and confirmed RSV infections in primary care, hospital, and pediatric intensive care unit admissions among infants aged 0 to 11 months, even in a season with a high community RSV burden, particularly for older infants. These findings are very useful for public health authorities to continue to implement immunization campaigns against RSV in the coming seasons. Moreover, universal immunization against RSV represents a transformative step towards reducing the burden of RSV in infants. With promising evidence from recent published studies, the expectations for a reduction of RSV-associated hospitalizations, alongside the improvement of public health outcomes and an equitable access to these measures, are high. However, achieving these goals will require addressing challenges related to vaccine uptake, funding, and RSV surveillance to prompt detect resistances due to mutations of the virus. These interventions need to be integrated into public health strategies because they hold the potential to make a significant impact on infant's health worldwide., Competing Interests: A. S-A. has received honorarium for attending scientific meetings from Sanofi and Pfizer. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (Thieme. All rights reserved.)

Background: In Catalonia, infants <6 months old were eligible to receive nirsevimab, a novel monoclonal antibody against respiratory syncytial virus (RSV). We aimed to analyze nirsevimab's effectiveness in hospital-related outcomes of the seasonal cohort (born during the RSV epidemic from October to January 2024) and compared them with the catch-up cohort (born from April to September 2023)., Methods: Retrospective cohort study of all infants born between October 1, 2023, and January 21, 2024, according to their immunization with nirsevimab (immunized and nonimmunized). We followed individuals until the earliest of an outcome-hospital emergency visits, hospital admission or pediatric intensive care unit (PICU) admission due to RSV-associated or all-causes bronchiolitis-death or the end of the study. We used the Kaplan-Meier estimator and fitted Cox regression models using a calendar time scale to estimate hazard ratios (HRs) and their 95% confidence interval (CI). Sensitivity analysis was performed through matching., Results: Among 15,341 infants, a dose of nirsevimab led to an adjusted HR for hospital admission, PICU admission and emergency visits due to RSV bronchiolitis of 0.26 (95% CI: 0.17-0.39), 0.15 (95% CI: 0.07-0.28) and 0.46 (95% CI: 0.23-0.90), respectively. For all-causes bronchiolitis, the former adjusted HRs were 0.45 (95% CI: 0.31-0.63), 0.23 (95% CI: 0.13-0.41) and 0.49 (95% CI: 0.35-0.68), respectively., Conclusions: Nirsevimab was associated with reductions of 74% and 85% hospitalizations and PICU admissions regarding RSV-associated bronchiolitis, respectively. These percentages are slightly lower than those for the catch-up cohort. This information may help the implementation of RSV-immunization campaigns by public health authorities., Competing Interests: The authors have no conflicts of interest to disclose., (Copyright © 2025 Wolters Kluwer Health, Inc. All rights reserved.)