Abstract

This report details current seasonal influenza circulation patterns as of early February 2026 and was prepared for the Northern Hemisphere VCM on February 20, 2026. 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: HA clade D.3.1.1 continues to grow globally, replacing C.1.9.3 and most other D.3.1 clades. Subclades within D.3.1.1 carrying HA1:139N or HA1:205K have started to grow in Europe and North America but remain at low frequencies elsewhere. Neutralization titers from human sera showed the highest antigenic advance for D.3.1.1, D.3.1:270A, and D.3.1.1:155E, while ferret serology showed little signal of antigenic advance for recent clades. We predicted that vaccine candidates from D.3.1.1 including A/Michigan/103/2025, A/Tokyo/EIS13-602/2025, and A/Tokyo/EIS13-863/2025 were antigenically closest to the predicted future H1N1pdm populations.

A/H3N2: HA clade K is the dominant clade globally. In all regions where K and J.2.3 have cocirculated recently (North America, South America, and Europe), J.2.3 has a consistently lower fitness than K and its emerging haplotypes. Both human and ferret serological data indicated similarly high antigenic advance of clades K, J.2.4, and J.2.3. A subclade of J.2.4 with HA1:135N had the highest antigenic advance in ferrets and humans. We predicted that vaccine candidates from K like A/Darwin/1415/2025 were closest to the predicted future H3N2 populations.

B/Vic: HA clade C.3.1 has circulated outside of the USA with most samples collected in Australia, Chile, and Brazil. However, we only estimate a consistently high growth advantage for C.3.1 in North America. A sibling clade of C.3.1 (C.3 with HA1:255P, 267V, and 197N substitutions) has reassorted with a different NA background and remains at low frequency. C.3.1 and other C.3 descendants with the HA1:197N substitution have the highest antigenic advance of any clade by ferret serological data.