Global influenza epidemiology after 2020: patterns of circulation, epidemic timing and duration, and implications for vaccination strategies

Introduction: There remains uncertainty whether global influenza seasonality, viral dynamics and epidemic duration have re-established after 2021. Aim: We describe global circulation patterns of influenza viruses from 2021 to 2025 and discuss implications for prevention and surveillance. Methods: We analysed World Health Organization (WHO) FluNet sentinel and non-sentinel/not-defined virological data from week 1/2021 to 26/2025, stratifying by latitude, region and season. We calculated influenza positivity rate, proportion of virus (sub)types, typical peak timing and duration of influenza epidemics (applying the 75% annual average percentage method). Results: Sentinel surveillance in 120 countries reported 500,870 detections in this period; positivity rate rose globally from 3.0% in 2021 to 23.7% in 2024. Type A viruses caused over two-thirds of cases, with variability across WHO Regions and seasons. Among A subtypes, A(H3N2) dominated in 2021/22 and A(H1N1)pdm09 in 2023/24, while all but three influenza B cases were B/Victoria. Epidemic peaks typically occurred from December to March and May to August in northern and southern hemispheres countries, respectively, while tropical countries showed highly heterogeneous timing. The median epidemic duration was ca 10 weeks above 30° north, and it varied between 15 and 30 weeks at more southern latitudes. Non-sentinel/not-defined feeds showed stronger A-skew and lower characterisation. Conclusion: Influenza virus circulation shows convergencetoward seasonal architectures described before the COVID-19 pandemic, although changes in lineage ecology and epidemic duration persist. Our results confirm the need for latitude-tailored vaccination schedules, consolidation of trivalent vaccines, and strengthened surveillance to better anticipate changes in influenza circulation and support preparedness.