Real-world effectiveness of nirsevimab immunisation against bronchiolitis in infants: a case–control study in Paris, France

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in infants. Nirsevimab, an RSV-neutralising monoclonal antibody, was approved for use in the EU in 2022, and a national immunisation campaign began in France in September, 2023. We aimed to assess the effectiveness of nirsevimab in reducing paediatric emergency department visits (and subsequent hospitalisations) for all-cause bronchiolitis and RSV-associated bronchiolitis.