RSV bronchiolitis: a disease only for those who do not receive prophylaxis

Bronchiolitis is a leading cause of hospitalization in infants, with RSV being the primary pathogen. In 2023, nirsevimab was approved for universal prophylaxis, demonstrating high effectiveness in reducing RSV-related hospitalizations. In Italy, nirsevimab was introduced in the vaccination schedule only during the current 2024-2025 epidemic season, with significant regional differences. In our region, Lazio, nirsevimab was introduced in late November 2024, leading only to a 43% overall reduction in RSV bronchiolitis hospitalizations compared to the previous season. The highest impact was observed in newborns immunized at birth (- 82.7%), while the catch-up group showed lower-than-expected benefits (- 29%). Immunized infants had significantly lower oxygen requirements (42.9% vs. 82.4% in non-immunized) and resulted to be approximately seven times more likely not to require oxygen compared to non-immunized patients.

Universal infant immunization against RSV marks a significant breakthrough in the natural history of bronchiolitis, and early and widespread nirsevimab administration appears to be crucial in minimizing RSV's hospitalizations.

• Nirsevimab was approved in 2023 for the universal prophylaxis against RSV bronchiolitis. • Nirsevimab was was introduced in the Italian vaccination schedule only during the current 2024-2025 epidemic season, with significant regional differences.

• In Lazio, Italy, the delayed introduction of nirsevimab in November 2024 and the reduced vaccination coverage of the catch-up group resulted in a lower-than-expected reduction in RSV bronchiolitis hospitalizations. • Early and widespread administration of nirsevimab is crucial to maximizing its benefits and reducing severe RSV cases.