Assessing the Effects of Nirsevimab on Respiratory Syncytial Virus Bronchiolitis Outcomes: Insights From A Two-Season Analysis.

OBJECTIVES

Nirsevimab was approved in Europe in 2022 for respiratory syncytial virus (RSV) prevention. In October 2023, Catalonia (Spain) launched a universal immunization program for infants under 6 months during their first RSV season. This study aims to assess the characteristics of patients admitted with bronchiolitis following nirsevimab implementation and to compare the features of the immunized patients across two consecutive seasons.

METHODS

This prospective, observational, single-center study included infants aged up to 12 months admitted for bronchiolitis between October 2023 and May 2025, comparing clinical and epidemiological data across two seasons (S1: October 2023-May 2024; S2: October 2024-May 2025).

RESULTS

574 patients were included (S1: 352; S2: 222), with a median age of 3.2 months (interquartile range [IQR]: 1.5-6.5). Most were previously healthy (437/574, 76.1%) and immunized with nirsevimab (406/574, 70.7%). RSV was detected in 215/574 (37.5%) of cases. 159/574 (27.7%) required Pediatric Intensive Care Unit (PICU) admission, and the median length of hospital stay was 5 days (IQR:3-8). In S2, immunization rates increased (S1:61.6% vs S2:85.1%, p = < 0.001) whereas RSV infections decreased (S1:41.2% vs S2: 31.5%, p = 0.020). Among immunized patients, S2 exhibited a higher proportion of healthy children (S1:65.0% vs S2:77.8%, p = 0.005), fewer PICU admissions (S1:33.2% vs S2:21.2%, p = 0.007) and shorter hospital stays (S1: 6 [IQR:4-9.5] days vs S2: 5 [IQR:3-7.5] days, p = 0.033). Furthermore, in multivariable logistic regression analysis, admission during the second season remained independently associated with a lower risk of PICU admission (adjusted OR 0.587, 95% CI 0.368-0.936; p = 0.025).

CONCLUSION

Among hospitalized infants with bronchiolitis after nirsevimab implementation, immunized patients admitted during the second season showed more favorable clinical outcomes, including lower PICU utilization and shorter hospitalizations.