Which influenza A subtypes were most prevalent in the 2024-2025 season globally and by region?
Executive summary
Global surveillance for the 2024–2025 influenza season points to a complex picture: influenza A viruses dominated overall, but the prevailing A subtype varied by place and time, with A(H3N2) rising globally in later reports while A(H1N1)pdm09 had dominated much of the 2024–25 northern‑hemisphere winter surveillance data [1] [2]. National datasets and peer‑reviewed analyses show both subtypes caused substantial burdens—some sources report H1N1 slightly more common in aggregate surveillance while others document recent surges in H3N2, especially in the Western Pacific and parts of southern Africa [3] [1].
1. Global picture: a season of two rivals
WHO’s global situation notes an overall increase in influenza activity and identifies A(H3N2) as the predominant circulating subtype in more recent months, driven by expansion of an A(H3N2) clade in parts of the world [1], yet WHO’s seasonal summary for October 2024–May 2025 reports that influenza A viruses predominated during that surveillance window and that A(H1N1)pdm09 predominated among subtyped viruses during the 2024–25 northern‑hemisphere winter season [2], a tension reflected in peer‑reviewed and national data that together show the global dominance shifted over the season rather than being fixed [3].
2. Northern Hemisphere and the United States: H1N1 led the winter surge, H3N2 emerging later
Multiple surveillance products and reviews indicate influenza A(H1N1)pdm09 was the majority of subtyped specimens during the 2024–25 northern‑hemisphere winter in many reporting systems, including WHO summaries and CDC national reports for that season [2] [4], while later U.S. weekly FluView updates through late 2025 show periods where A(H3N2) became the most frequently reported subtype in subsequent seasons or weeks [5] [6], underscoring that subtype proportions changed over time within national surveillance streams [4] [5].
3. Western Pacific and southern Africa: clear H3N2 signals
WHO highlights the expansion of an A(H3N2) subclade that accounted for a large share of sequences submitted from the Western Pacific Region and documents near‑exclusive A(H3N2) detections during the South African peak in May 2025, signaling strong regional H3N2 activity even where earlier months had different patterns [1].
4. Regional nuance in Europe, China and other locales
European surveillance summaries from 2023–24 show influenza A dominance with substantial H1N1 representation in prior seasons [7], and hospital‑based studies in China for 2024–25 report that H1N1 and H3N2 were both predominant among clinical cases, indicating co‑circulation of the two A subtypes rather than a single universal winner [8] [9]. Peer‑reviewed syntheses and pediatric analyses also record mixed subtype shares—one clinical review cited CDC data showing H1N1 accounted for 53.7% and H3N2 46.3% of reported cases in some compiled datasets—reinforcing that aggregate proportions depend on which surveillance streams and time slices are combined [3] [10].
5. What the data limitations mean for interpretation
Comparing “most prevalent” across global and regional sources requires caution because surveillance coverage, subtyping rates, and reporting windows differ: WHO’s summaries, national FluNet submissions, hospital cohorts and literature reviews each sample different populations and periods, which can make A(H1N1)pdm09 appear dominant in one dataset and A(H3N2) in another depending on timing and geography [2] [1] [8]. Where sources disagree, the reporting documents themselves note shifting dominance over the season rather than a single, uniform global predominance [1] [2].
6. Bottom line
The 2024–2025 season was characterized by influenza A dominance with both A(H1N1)pdm09 and A(H3N2) playing leading roles: A(H1N1)pdm09 predominated during much of the 2024–25 northern‑hemisphere winter surveillance window in several reporting systems [2] [4], while A(H3N2) rose to predominate in more recent global and regional reports—notably in the Western Pacific and parts of southern Africa—revealing a geographically and temporally heterogeneous season [1] [8].