Impact of RSV Preventive Strategies on Hospitalizations for Bronchiolitis in Infants

INTRODUCTION: Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and hospitalization in infants. Since 2023, France has implemented two preventive strategies aimed at reducing RSV burden: long-acting monoclonal antibody nirsevimab and RSV vaccine for pregnant women, the latter becoming available from September 2024. This study aimed to assess the impact of RSV preventive strategies on the clinical severity and viral epidemiology of bronchiolitis in hospitalized infants. MATERIAL AND METHODS: We conducted a retrospective study at Armand Trousseau University Hospital, including infants under 12 months hospitalized for a first episode of bronchiolitis between October and March during four winter seasons (2021-2025). Clinical and virological data were compared between the pre-intervention (2021-2023) and post-intervention (2023-2025) periods. Subgroup analyses were performed based on RSV infection and prophylaxis status. RESULTS: A total of 509 infants were included (292 pre-intervention, 217 post-intervention). Post-intervention infants were older (median, 3.2 vs. 1.9 months, p = 0.001), required less oxygen therapy (58.5% vs. 70.9%, p < 0.001), and had shorter hospital stays (median, 3.0 vs. 5.0 days, p < 0.001). Compared with untreated infants (n = 140), those who received nirsevimab (n = 67) had significantly lower rates of RSV infection and viral co-infections (OR: 0.15 (95%CI: 0.08-0.29), p < 0.001; and 40.4% vs. 25.0%, p = 0.035), while human metapneumovirus was detected more frequently (OR: 11.00 (95%CI: 3.50-34.55), p < 0.001). DISCUSSION AND CONCLUSION: During the period following the introduction of RSV preventive strategies, a reduced hospital burden, milder clinical courses, and shifts in viral epidemiology were observed. These findings support the continued implementation of RSV immunoprophylaxis and underscore the importance of ongoing surveillance to monitor its long-term impact.