Introduction of nirsevimab in Catalonia, Spain: description of the incidence of bronchiolitis and respiratory syncytial virus in the 2023/2024 season.

Respiratory syncytial virus (RSV) causes most of the cases of bronchiolitis and thousands of deaths annually, particularly in infants less than 6 months old. 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. We collected epidemiological data from the Information System for Surveillance of Infections in Catalonia (SIVIC) 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 0-11 m-old with respect to 12-35 m-old between September 2014 and January 2024. We analysed the RR of the incidence of RSV-confirmed infection for 0-11 m-old and 12-35 m-old with respect to the > 35 m-old, from January 2021 to January 2024. We then computed the relative difference of the RR, designated as percentage of reduction of risk, between season 2023/2024 and former epidemics. With a global coverage recorded rate for nirsevimab of 82.2% in January 2024, the age-specific 0-11 m-old RR (95% CI) of RSV infection incidence for > 35 m-old was 1.7 (1.5-2.0) in season 2023/2024. 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 2020/2021, 2021/2022, and 2022/2023, respectively. Regarding the incidence of all-causes bronchiolitis for the 0-11 m-old group compared to the 12-35 m-old, the pre-pandemic (2014/2015-2019/2020) and 2022/2023 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 most recent season, 2023/2024. Conclusion: Concurring with the introduction of nirsevimab, the risk of RSV infection for infants aged 0-11 m-old compared to > 35 m-old has been reduced by 75.6% (73.4-77.5) in last season, and the risk for all-causes bronchiolitis for 12-35 m-old by 61.9% (60.9-62.9) from the pre-pandemic period and by 39.8% (39.3-40.2) from the 2022/2023 epidemic, despite high RSV community transmission, especially in older infants What is Known: • RSV is responsible for approximately 70% of bronchiolitis cases and causes severe disease, particularly in infants < 6 months of age. • Nirsevimab effectiveness against RSV-associated disease, particularly hospitalisations, was expected to be around 80%; other Spanish regions, such as Galicia and Valencia, and European countries including Luxembourg and Germany, have already reported good results in implementing nirsevimab to prevent RSV-associated hospitalisations and PICU stays. What is New: • We provide insight into the community incidence of RSV and all-causes bronchiolitis for season 2023/2024, when nirsevimab has been introduced to the Catalan population, using.   primary healthcare data, which enabled us to assess the burden of RSV infections and bronchiolitis in the commonly seasonally saturated primary healthcare practices. • Our study reveals that the risk of all-causes bronchiolitis for infants aged 0-11 m-old compared to older infants was reduced by 40% compared to the previous season and 62% compared to pre-pandemic standards, and for RSV infection it was reduced by 76%.