Nirsevimab Immunisation Significantly Reduces Respiratory Syncytial Virus-Associated Bronchiolitis Hospitalisations and Alters Seasonal Patterns

Aim This work was performed to assess the impact of nirsevimab immunisation on acute bronchiolitis hospitalisations during nearly the entire 2023-2024 epidemic year. Methods An observational, multicentre, prospective study was conducted from 1 September 2021 to 15 June 2024 across 20 hospitals in two Spanish regions. Infants up to 12 months old admitted for acute bronchiolitis were included. Demographic, clinical and microbiological data were analysed across three epidemic years (2021-2022, 2022-2023 and 2023-2024). Statistical analyses were performed to evaluate the effectiveness of nirsevimab in preventing respiratory syncytial virus (RSV)-associated hospitalisations. Results In total, 2656 patients were included. Bronchiolitis hospitalisations significantly declined in the post-nirsevimab epidemic year compared with previous years. The 2023-2024 season displayed a bimodal distribution, with the first peak dominated by RSV cases and the second by rhinoviruses and metapneumovirus. The proportion of RSV-associated bronchiolitis hospitalisations decreased by 20%-30%, while rhinovirus- and metapneumovirus-associated bronchiolitis cases increased by 10%-20%. The effectiveness of nirsevimab in preventing RSV-associated admissions was estimated to be approximately 70%. Conclusion Nirsevimab immunisation significantly reduced RSV-associated bronchiolitis admissions, though an increase in rhinovirus- and metapneumovirus-associated cases was observed.