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 compared 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 (Figure 1).
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.
