JAMA Netw Open. 2025 Oct 1;8(10):e2540720. doi: 10.1001/jamanetworkopen.2025.40720.
ABSTRACT
IMPORTANCE: New resistant mutations of respiratory syncytial virus type B (RSV-B) have been observed during the first year of implementation nirsevimab treatment. During the second season of implementation of nirsevimab treatment in France, RSV-B was predominant.
OBJECTIVES: To assess and compare the effectiveness of nirsevimab treatment in preventing RSV bronchiolitis in pediatric emergency departments during the first and second seasons of implementation of treatment in France.
DESIGN, SETTING, AND PARTICIPANTS: This a multicenter test-negative case-control study included 636 infants younger than 1 year who received a diagnosis of a first bronchiolitis episode in 5 pediatric emergency departments in France during the 2 first seasons of implementation of nirsevimab treatment (from October 5, 2023, to February 29, 2024, and from October 15, 2024, to January 31, 2025) and underwent a nasopharyngeal test for RSV.
MAIN OUTCOMES AND MEASURES: The main outcome of the study was the RSV status of the bronchiolitis cases. Multivariable logistic regression was performed with nirsevimab as the explanatory variable, adjusted on age, sex, risk factors of bronchiolitis, type of childcare, month, and center of inclusion. Effectiveness was calculated for each season and compared using the likelihood ratio test. Subgroup analysis by age and severity as well as sensitivity analyses were performed.
RESULTS: The study included 636 patients with bronchiolitis (median age, 3.0 months [IQR, 1.4-5.0 months]; 333 boys [52.4%]). In both seasons, 162 of 636 patients (25.5%) were immunized with nirsevimab. During the first season of implementation, 273 of 384 patients (71.1%) tested positive for RSV; during the second season, 181 of 252 patients (71.8%) tested positive for RSV. The effectiveness of nirsevimab treatment against RSV bronchiolitis was estimated to be 83.2% (95% CI, 68.0%-91.4%) during the first season and 89.3% (95% CI, 77.8%-95.1%) during the second season; no statistically significant difference in effectiveness was found between the 2 seasons (P = .97). Subgroup and sensitivity analyses provided similar results.
CONCLUSIONS AND RELEVANCE: In this test-negative case-control study of nirsevimab treatment, its effectiveness in reducing pediatric emergency department visits for RSV bronchiolitis during the second season of national immunization was high and comparable with that observed in the first season. Although RSV-B resistant strains had been recently identified, it did not appear to have important clinical consequences to date. Continued close monitoring of RSV epidemiology in the context of the widespread nirsevimab use remains essential.
PMID:41165704 | DOI:10.1001/jamanetworkopen.2025.40720
