Abstract

This report details current seasonal influenza circulation patterns as of early September 2026 and was prepared for the Southern Hemisphere VCM on September 18, 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 is now the dominant clade globally. D.3.1.1 subclades with HA1 139N and 155E are growing rapidly and show strong antigenic advance in human and ferret serology. Based on human serology, we predicted that vaccine candidates from D.3.1.1 with 139N and 155E (A/Tokyo/EIS15-043/2026 and A/Sydney/81/2026) were antigenically closest to predicted future H1N1pdm populations.

A/H3N2: HA clade K has fixed globally. Multiple emerging variants within K appear to have higher growth rates than ancestral K viruses. Variants of K with HA1 223I or 79I/209Q show the most antigenic advance in human serology. A K:328T variant reflects a high fitness reversion to a pre-K allele with no clear antigenic effect. From human serology, we predicted that the current Northern Hemisphere vaccine, A/Darwin/1415/2025, was among the closest strains to predicted future H3N2 populations.

B/Vic: HA diversity has persisted with C.3 and C.5 descendants co-circulating and diversifying. However, C.3.1, C.3.3, C.5.1, and C.5.7 all appear to be declining in frequency, while C.5.6 variants like C.5.6.1 with HA1 241Q are growing. C.3 descendants with HA1 197N have the highest antigenic advance in ferrets followed by C.5.6.1 variants with 241Q.