Impact of national RSV immunization programme on the epidemiology of bronchiolitis in Greece

Abstract

Universal RSV immunization strategies for infants, including monoclonal antibody administration and maternal vaccination during pregnancy, were implemented in Greece for the first time during the 2025–2026 season. We conducted a prospective observational study of infants aged <12 months hospitalized with bronchiolitis at a quaternary paediatric hospital in Greece between November 2025 and April 2026. Bronchiolitis incidence was compared with that of the 2023–2024 and 2024–2025 seasons. Severity of bronchiolitis was assessed using the modified Tal score. Across the three seasons, 854 of 15246 admissions were due to bronchiolitis. The incidence decreased significantly in 2025–2026 to 42 per 1000 admissions, compared to 64 and 59 in 2024–2025 and 2023–2024, respectively. The 2025–2026 epidemic curve of bronchiolitis was flatter and more prolonged, with a delayed peak in March. Of 186 infants with bronchiolitis, 19.9% had received RSV immunization. Disease severity and oxygen requirements did not differ significantly between immunized and unimmunized infants, although unimmunized infants were older and had a trend towards higher complication rates. In conclusion, national RSV immunization was associated with reduced bronchiolitis hospitalizations and complications. However, the smaller-than-expected reduction in disease rates underscores the need to improve awareness, coverage, and timely immunization.