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

Background 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. Methods In this case-control study in a paediatric emergency department in Paris, France, we included all infants aged 12 months or younger who attended the department between Oct 14, 2023, and Feb 29, 2024, and whose nirsevimab status was known. Infants were classed as cases if they had all-cause bronchiolitis; all other infants were classed as controls. The primary outcome was the effectiveness of nirsevimab against paediatric emergency department visits for all-cause bronchiolitis during the 2023-24 RSV season. Secondary outcomes were paediatric emergency department visits for RSV-associated bronchiolitis; hospitalisations for all-cause bronchiolitis, RSV-associated bronchiolitis, and severe RSV-associated bronchiolitis requiring supplemental oxygen or feeding by nasogastric tube; and severe RSV-associated bronchiolitis requiring admission to the paediatric intensive care unit. Effectiveness estimates were adjusted for age, week of paediatric emergency department visit, and sex. Findings Our study included 2786 infants, 864 with all-cause bronchiolitis (cases) and 1922 without bronchiolitis (controls). 178 (21%) of the 864 cases had received nirsevimab, and 305 (35%) cases were hospitalised for all-cause bronchiolitis. 200 (72%) of the 277 cases tested for RSV were positive, of whom 22 (11%) had received nirsevimab. 701 (36%) of 1922 infants in the control group had received nirsevimab. The effectiveness of nirsevimab against paediatric emergency department visits for all-cause bronchiolitis was 47% (95% CI 33-58). Nirsevimab effectiveness was 83% (71-90) against paediatric emergency department visits for RSV-associated bronchiolitis, 59% (42-71) against hospitalisations for all-cause bronchiolitis, 83% (72-90) against hospitalisations for RSV-associated bronchiolitis (91% [78-96] against those necessitating supplement oxygen and 88% [74-95] against those necessitating feeding via a nasogastric tube). Nirsevimab did not significantly reduce admissions to the paediatric intensive care unit (67% [95% CI -100 to 95]). Interpretation During the first French national immunisation campaign, a single dose of nirsevimab effectively reduced paediatric emergency department visits (both all-cause visits and visits related to RSV-associated bronchiolitis) and subsequent hospitalisations.