<title>Abstract</title> Purpose Respiratory Syncytial Virus (RSV) causes most of the bronchiolitis and thousands of deaths annually, particularly in infants less than 6 months. In Catalonia (Spain), infants born between April 2023 and March 2024 aged 0–6 months during their first RSV season have been candidates to receive nirsevimab, the novel monoclonal antibody against RSV, since October 2023. We aimed to analyse the dynamics of all-causes bronchiolitis diagnoses and RSV community infections in the current season and compare them to pre-nirsevimab epidemics. Methods We collected epidemiological data from the Information System for Surveillance of Infections (SIVIC) in Catalonia on daily all-causes bronchiolitis clinical diagnoses and RSV-confirmed cases provided by rapid antigen tests in Primary Care Practices. We calculated the rate ratio (RR) for the incidence of all-causes bronchiolitis for children aged 0-11m-old concerning 12-35m-old between September 2014 and January 2024. We analysed the RR of the incidence of RSV-confirmed infection for 0-11m-old and 12-35m-old concerning the &gt; 35m-old, from January 2021 to January 2024. We then computed the relative difference of the RR, named percentage of reduction of risk, between season 2023/24 and former epidemics. Results With a global coverage rate for nirsevimab of 82.2% in January 2024, the age-specific 0-11m-old RR (95%CI) of RSV infection incidence regarding &gt; 35m-old was 1.7 (1.5-2.0) in last season 2023/24. The RR (95%CI) had been 7.4 (5.6–9.9), 8.8 (6.9–11.3), and 7.1 (5.7–8.9) in 2020/21, 2021/22 and 2022/23 seasons. Regarding the incidence of all-causes bronchiolitis for 0-11m-old group compared to the 12-35m-old, the pre-pandemic (2014/15-2019/20) and 2022/23 RR (95%CI) were 9.4 (9.2–9.6) and 6.0 (5.7–6.2) respectively, significantly higher than the RR of 3.6 (3.4–3.8) for the last season 2023/24. Conclusions Coinciding with the introduction of nirsevimab, the risk of RSV infection for infants aged 0-11m-old regarding &gt; 35m-old has been reduced by 75.6% (73.4–77.5) from last season, and the risk for all-cause bronchiolitis regarding 12-35m-old by 61.9% (60.9–62.9) from the pre-pandemic period and by 39.8% (39.3–40.2) from 2022/23 epidemic, despite a high RSV community transmission, especially in older infants.

Introduction of nirsevimab in Catalonia, Spain: description of bronchiolitis and the Respiratory Syncytial Virus incidence in the 2023/24 season