Background: Bronchiolitis represents a major cause of infant hospitalization, predominantly due to respiratory syncytial virus (RSV). Nirsevimab, a long-acting monoclonal antibody targeting the prefusion RSV F protein, has recently been introduced to provide season-long protection against RSV-related lower respiratory tract infection. Methods: We conducted a multicenter, retrospective, population-based study across the Liguria Region (Northwest Italy) to assess the impact and effectiveness of the 2024–2025 nirsevimab immunization campaign. All infants under two years of age hospitalized with acute bronchiolitis between October 2023 and March 2025 were identified through the regional pediatric Hub-and-Spoke network. Data were compared between the pre-nirsevimab (2023–2024) and post-nirsevimab (2024–2025) seasons. Nirsevimab effectiveness (VE) against RSV-positive hospitalization was estimated using a test-negative design. Results: A total of 858 infants were hospitalized with bronchiolitis across the two seasons. Following the introduction of nirsevimab, bronchiolitis hospitalizations decreased from 41.7 to 17.3 per 1000 admissions (Incidence Rate Ratio 0.4; 95% CI: 0.4–0.5; p<0.001), corresponding to a 60% reduction. Estimated nirsevimab effectiveness was 65.0% (95% CI: 39.2–79.8%) against RSV-positive hospitalization. No high-risk infants—those previously eligible for palivizumab—developed severe RSV infection requiring PICU admission. Breakthrough infections occurred in a minority of immunized infants but showed a similar clinical course to non-immunized cases. Conclusions: The introduction of universal nirsevimab immunization in Liguria was associated with a marked reduction in RSV-related hospitalizations, confirming its strong real-world epidemiological impact. These findings support nirsevimab as an effective and feasible public health intervention to substantially reduce the burden of RSV bronchiolitis in infants.

Abstract: Aim: Recent literature has shown epidemiological changes in bronchiolitis with an increased incidence in the post-SARS-CoV-2 pandemic period but reports regarding disease severity are conflicting. We aimed to describe the epidemiology, disease severity, and microbiology of bronchiolitis during the 2022-2023 cold season compared to the previous five years. Methods: This single-center retrospective observational study at IRCCS Gaslini, Italy, included all children aged 0-2 years hospitalized for bronchiolitis from September 1st, 2017, to August 31, 2023. Findings from the 2022-2023 season were compared to the previous five years. Results: We observed a statistically significant increase in the 2022-2023 season in the absolute number of bronchiolitis admissions. Children who required mechanical ventilation dramatically increased from a total of 7 patients in the previous five seasons to 17 in the 2022-2023 season alone ( p=0.001). All other severity parameters significantly increased: the need for respiratory support ( p=0.002) , the median length of stay (5 vs. 4, p=0.001), and the median duration of respiratory support (4 vs. 3, p=0.016). Conclusions: We report a substantial increase in the severity of bronchiolitis in the season 2022-2023 with a remarkable number of previously healthy infants requiring mechanical ventilation. Our data represents a call in our setting for urgent measures to plan prophylactic and therapeutic strategies and prepare healthcare systems. Further studies are needed to confirm whether our findings are an isolated phenomenon or part of a true global trend. Health systems need to be prepared and protective preventive measures should be implemented for all newborns.

Objective: SARS-COV-2 pandemic had a profound impact on acute bronchiolitis epidemiology, especially in RSV diffusion and the burden of disease with implications on the management of prophylaxis and health resources. We aimed to compare clinical and epidemiological characteristics of bronchiolitis before and during the SARS-CoV-2 pandemic.   Methods: We conducted an observational study involving children aged 0-2 years with bronchiolitis admitted to a tertiary children’s hospital during the last 5 years. Demographic, clinical, microbiological, and outcome data were collected. Comparison between pre- and post-pandemic period, RSV positive versus non-RSV patients, and SARS-CoV-2 positive vs negative patients were carried out.    Results: A total of 647 patients admitted for bronchiolitis were included (264 female,40.8%, median age 78 days). Molecular diagnostic tests were performed in 617 patients (95.4%) with RSV detected in 62.5% of patients. SARS-CoV-2 was found in 3.9% of hospitalized bronchiolitis (3.9%). We observed a progressive increase in bronchiolitis admissions and a statistically significant increase over the years in the need for respiratory support. Conversely, this was not true for mechanical ventilation, duration of respiratory support, ICU admission, and length of stay. During the pandemic period children with prematurity increased, although only 1 child had an indication for prophylaxis. Discussion: We confirm the stronger impact of bronchiolitis in the 2021-22 season, which is likely explained by the higher prevalence of RSV and the immunity debt theory. However, our findings were conflicting in terms of worsening clinical severity. The increase of children with prematurity and the inter-seasonal spread of RSV highlight the importance of epidemiological surveillance systems that monitor RSV circulation.