Background: Acute bronchiolitis is one of the leading causes of hospitalization in infants, most frequently caused by respiratory syncytial virus (RSV). In 2024/25, Portugal implemented universal immunization with the monoclonal antibody Nirsevimab.The aim of this study was to assess the impact of Nirsevimab on the incidence, clinical severity, and hospitalization rates of RSV-related acute bronchiolitis in a Portuguese Secondary Care Hospital.Method: A retrospective study was conducted, including children younger than 24 months, seen in a Pediatrics Department, with a clinical diagnosis of acute bronchiolitis and laboratory-confirmed RSV infection (positive multiplex polymerase chain reaction [BioFire®] from nasopharyngeal secretions). Two periods were compare,d covering two epidemiological seasons: October 2023–March 2024 (pre-Nirsevimab) and October 2024–March 2025 (post-Nirsevimab). Clinical severity was assessed using the Bronchiolitis Score of Sant Joan de Déu. Statistical analysis was performed with JASP® version 0.19.3.0.Results: A total of 137 children were included, with a 24.1% reduction in cases between the two seasons (n=85 vs. n=52; p<0.001). The seasonal peak in both periods occurred in December, with 54.1% (n=46) of cases in 2023/24 and 42.3% (n=22) in 2024/25, which also corresponded to the highest number of moderate/severe cases (n=30 vs. n=14, respectively). In 2024/25, 26.9% (n=14) had received nirsevimab; 9 of these required hospitalization. In 75.2% of cases (n=103), mothers of children with acute RSV bronchiolitis had not received the RSV vaccine (Abrysvo®), while vaccination status was unknown in 24.8% (n=34). RSV was identified as the sole pathogen in 41.6% (n=57) of cases and as part of viral co-infections in 58.4% (n=80).Among co-detections, rhino/enterovirus was the most frequent agent (72.9% vs. 75%), followed by adenovirus (18.8% vs. 25%) and Influenza A (8.3%) in 2023/24 and Influenza B (15.6%) in 2024/25. Between the two periods, the median age increased [3.9 months (IQR 1.8–7.7) vs. 7.2 months (IQR 5.1–13.4), p<0.001], with male predominance in both (56.5% vs. 55.8%). Although overall hospitalization rates did not differ significantly (60.0% vs. 50.0%; p=0.25), there was a reduction in the proportion of hospitalized infants younger than 6 months (70.6% vs. 42.3%; p=0.016), in length of stay [5.0 (IQR 3.0–7.0) vs. 4.0 days (IQR 3.0–4.0); p=0.04], and in moderate/severe cases (69.4% vs. 51.9%; p=0.046).Oxygen therapy was required in 51.8% (low-flow) and 4.7% (high-flow nasal cannula) vs. 38.5% and 1.9% of cases, respectively, with reduced duration [4.0 (IQR 2.0–6.3) vs. 3.0 days (IQR 1.8–3.0), p=0.04]. One transfer to a Pediatric Intensive Care Unit occurred in 2023/24, and none in the following season. (table)Conclusion: Following Nirsevimab introduction, there was a reduction in incidence, length of hospital stay, duration of oxygen therapy, and clinical severity of RSV-related acute bronchiolitis, particularly among infants under 6 months of age.
