Core Insights - The report recommends the composition of influenza virus vaccines for the Northern Hemisphere for the 2025-2026 season, based on the WHO's technical consultation meetings held in February and September each year [1][28] - The recommended vaccine components include strains similar to A/Victoria/4897/2022 (H1N1)pdm09, A/Croatia/10136RV/2023 (H3N2), and B/Austria/1359417/2021 (B/Victoria lineage) for trivalent vaccines [30][31] - The report highlights the ongoing global circulation of A(H1N1)pdm09 and A(H3N2) viruses, with specific genetic lineages identified, indicating the need for continuous monitoring and adaptation of vaccine formulations [25][26] Seasonal Influenza Activity - Influenza activity from September 2024 to January 2025 showed a decrease in overall activity compared to the previous season, with variations in dominant viruses across different regions [3][4] - In Africa, influenza activity peaked in December, primarily driven by A(H3N2) detections, while in Asia, A(H1N1)pdm09 was the dominant strain during the reporting period [3][6] - The Americas experienced increased influenza activity starting in November, with A(H1N1)pdm09 and A(H3N2) viruses circulating at similar levels [8] Virus Characteristics and Antigenic Properties - The A(H1N1)pdm09 virus has been identified in various regions, with genetic analysis revealing the presence of lineages 5a.2a and 5a.2a.1, which continue to diversify [14][25] - A(H3N2) viruses predominantly belong to the 2a.3a.1 lineage, with significant mutations observed, particularly in the J.2 subclade [18][26] - The report indicates that B/Victoria lineage viruses are the only circulating B viruses, with all identified strains belonging to the 3a.2 lineage [22][27] Recommendations for Vaccine Composition - For the 2025-2026 Northern Hemisphere season, the WHO recommends specific strains for both egg-based and cell culture-based vaccines, emphasizing the need to exclude B/Yamagata lineage components due to the absence of circulating strains [30][31] - The report underscores the importance of using updated vaccine strains to enhance effectiveness against circulating viruses, as evidenced by serological studies showing good recognition of recent strains by vaccine-induced antibodies [25][27]