Age-related distribution of influenza B and vaccine match in the 2024-2025 season: A multi-season comparative analysis

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OBJECTIVES: The 2024-2025 season had the highest intensity of influenza-like illness in 13 seasons, and severe influenza B disease was reported by hospital physicians among children. These and the absence of influenza B/Yamagata after the COVID-19 pandemic prompted the evaluation of the 2024-2025 influenza cases by age group and influenza type, compared with previous seasons. METHODS: Influenza-positive hospital and community cases for the 2024-2025 season and four previous seasons with influenza B circulation were retrieved from several data sources. Raw numbers and relative percentages of cases, including intensive or enhanced care unit, and 30-day mortality were evaluated by age group and influenza type. Molecular characterization of influenza viruses of the 2024-2025 season was compared with other seasons. RESULTS: The 2024-2025 B/Victoria demonstrated molecular resemblance to the 2024-2025 vaccine. During the 2024-2025 season and other seasons with circulating and vaccine influenza B/lineage match, influenza B predominantly affected younger age groups. During seasons with circulating and vaccine influenza B/lineage mismatch, the percentage of influenza B cases among older age groups was not substantially different that the younger age groups. Influenza vaccination rates among children have been low throughout the evaluation period. CONCLUSIONS: Increasing influenza vaccination among young age groups is paramount for reducing influenza morbidity.

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