Abstract

This report details current seasonal influenza circulation patterns as of mid-September 2025 and was prepared for the VCM ahead of the South Hemisphere VCM on September 19, 2025. This is not meant as a comprehensive report, but is instead intended as particular observations that we’ve made that may be of relevance. Please also note that observed patterns reflect the GISAID database and may not be entirely representative of underlying dynamics. All analyses are based on the Nextstrain pipeline with continual updates posted to nextstrain.org/seasonal-flu. In compliance with data sharing agreements, this public version of the report does not include raw serological measurements.

A/H1N1pdm: D.3 continues to sweep globally likely due to reassortment with different NA backgrounds. We define the new HA clade D.3.1 to track this reassortant group. This clade continues to diversify genetically with at least one major subclade rising to high frequency in Europe, carrying 113K, 139D, and 283K substitutions. A/Wisconsin/67/2022 effectively covers recent viruses except those carrying HA1:155E or 155R substitutions. Our prediction models suggest that vaccine candidates in the D.3.1 subclade bearing 113K represent the closest antigenic match to the future population.

A/H3N2: J.2.4 (with HA1 189R and 135K) is growing rapidly in Africa, Europe, North America, and Oceania, shows strong antigenic drift in ferrets, and is predicted to fix in these regions. J.2 with 145N and 261Q is growing rapidly in Southeast Asia and shows strong antigenic drift in humans. J.2.4, J.2.3 (189R and 158K), and J.2.5 (145N and 158K) are poorly covered by A/DistrictOfColumbia/27/2023 and A/Croatia/10136RV/2023. We predict J.2.4 vaccine candidates like A/Sydney/1359/2024, A/Nepal/N042/2025-egg, and A/Singapore/GP20238/2024-egg are best matched to the future population, although sera to these strains poorly cover other circulating viruses.

B/Vic: The subclade C.3.1 is growing in North America with a strong signal of antigenic drift from ferret HI data associated with an HA1 197N substitution. The newly defined subclade C.3.1 shows evidence of reassortment to the NA background of HA subclade C.5.1. C.5.7 and C.5.1 appear to be declining globally as C.5.6.1 grows. Sera against B/Pennsylvania/14/2025 appears to cover C.3.1 and other extant clades.