Objectives: Nirsevimab was approved in Europe in 2022 for respiratory syncytial virus (RSV) prevention. In October 2023 Catalonia, a region of Spain, launched a universal immunization program for infants under 6 months during their first RSV season. The aims are assessing the characteristics of patients admitted with bronchiolitis following nirsevimab’s implementation and comparing the features of the immunized patients during two seasons. Methods: This prospective, observational, single-center study included patients aged up to 12 months old 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 (406/574, 70.7%). RSV was detected in 215/574 (37.5%) of cases. 159/574 (27.7%) required ICU 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-value=<0.001) and RSV infections decreased (S1:41.2% vs S2: 31.5%, p-value=0.020). Among immunized patients, S2 showed a higher proportion of healthy children (S1:65.0% vs S2:77.8%,p-value=0.005), fewer ICU admissions (S1:33.2% vs S2:21.2%,p-value=0.007) and shorter hospital stays (S1: 6 [IQR:4-9.5] days vs S2: 5 [IQR:3-7.5],p-value=0.033). Conclusion : Following nirsevimab implementation, immunized patients in the second season had milder outcomes, including fewer ICU admissions and shorter hospital stays.

Background Several clinical trials have shown that nirsevimab, an antibody targeting the respiratory syncytial virus (RSV), reduces RSV-bronchiolitis requiring admission. In 2023-2024, Catalonia and Andorra adopted immunization strategies for children < 6 months and those born during the epidemic season. This study evaluates the effectiveness of nirsevimab in preventing hospitalizations from RSV bronchiolitis. Methods In the epidemic season of 2023-2024, a test-negative case-control study was conducted in three hospitals from Catalonia and Andorra. Patients <12 months old admitted with bronchiolitis and tested for RSV using molecular microbiology tests were included. The effectiveness in preventing RSV-bronchiolitis hospitalization and severe disease was estimated using multivariate models. Comparisons between immunized, non-immunized and non-eligible patients were made in prospectively collected epidemiological, clinical, and microbiological variables. Results 234 patients were included. RSV was detected in 141/234(60.2%), being less common in the immunized group (37% vs 75%, p<0.001). The rate of immunized patients among those eligible was 59.7%. The estimated effectiveness for RSV-associated lower respiratory tract infection was 81.0% (95% confidence interval: 60.9-90.7), and for preventing severe disease (the need for NIV/CMV), 85.6% (41.7-96.4%). No significant differences by immunization status were observed in patients with RSV concerning viral co-infections, the need for NIV/CMV or length of hospital stay. Conclusions This study provides real-world evidence of the effectiveness of nirsevimab in preventing RSV-LRTI hospitalization and severe disease in infants during their first RSV season following a systematic immunization program. Immunized patients did not exhibit a higher rate of viral co-infections nor differences in clinical severity once admitted.