TOPLINE:

Nirsevimab remained highly effective in reducing paediatric emergency department visits for respiratory syncytial virus (RSV)-associated bronchiolitis during the second season of national immunisation in France, with effectiveness comparable to that observed during the first season.

METHODOLOGY:

• Researchers in France conducted a multicentre test‑negative case-control study to evaluate the effectiveness of nirsevimab in preventing RSV‑associated bronchiolitis at paediatric emergency departments during first (October 2023 to February 2024) and second (October 2024 to January 2025) seasons of treatment implementation.
• They included 636 infants younger than 1 year (median age, 3 months; 52.4% boys) who presented with a first episode of bronchiolitis and underwent nasopharyngeal testing for RSV.
• Infants with RSV‑positive bronchiolitis were classified as cases, and those with RSV‑negative bronchiolitis served as control patients.
• The main outcome was the proportion of bronchiolitis cases testing positive for RSV during each season.

TAKEAWAY:

• Overall, 25.5% of infants received nirsevimab; RSV positivity was 71.1% during the first season and 71.8% during the second.
• Nirsevimab immunisation was 83.2% effective (95% CI, 68.0%-91.4%) in preventing RSV-associated bronchiolitis during the first season and 89.3% effective (95% CI, 77.8%-95.1%) during the second season.
• No significant difference in the effectiveness of nirsevimab was observed between two seasons (P = .97), indicating consistent protection across both seasons.
• The effectiveness of nirsevimab remained high for preventing hospitalisation — 82.3% during the first season and 89.6% during the second season.

IN PRACTICE:

"By showing that the effectiveness of treatment during the second season is as strong as during the first, our study suggests that the widespread use of nirsevimab did not erode its effectiveness," the authors of the study wrote.

SOURCE:

This study was led by Lea Lenglart, MD, Robert Debré Hospital, Assistance Publique-Hôpitaux de Paris, Paris, France. It was published online on October 30, 2025, in JAMA Network Open.

LIMITATIONS:

Genomic sequencing of RSV strains was not performed. The observational nature of the study precluded causal inference. The possibility of false‑negative results cannot be ruled out, particularly with rapid antigen testing compared with polymerase chain reaction.

DISCLOSURES:

This study was funded by Sanofi and AstraZeneca. Some authors reported receiving non-financial support, personal fees, and grants and having other ties with multiple pharmaceutical companies.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.