December 1, 2025

Extended influenza seasons in Australia and New Zealand in 2025 due to the emergence of influenza A(H3N2) subclade K viruses.

Key Points

Extended influenza season occurred due to outbreaks of subclade K virus, impacting Australia and New Zealand.
Subclade K viruses differ genetically from the 2025 A(H3N2) vaccine strain, raising concerns for seasonal effectiveness.
Observational analysis identified subclade K's spread across 34 countries, excluding South America, suggesting global reach.
Further expansion of the subclade K virus is likely during the 2025/26 northern hemisphere winter season.

Abstract

In Australia and New Zealand, late outbreaks of an A(H3N2) variant virus termed subclade K extended the 2025 influenza season. Subclade K viruses were genetically and antigenically distinct from the 2025 vaccine A(H3N2) strain A/Croatia/10136RV/2023 (H3N2)-like virus and previously circulating subclade J viruses. Subclade K viruses have since been detected in over 34 countries and appear to have spread globally, except in South America. It is thus likely that they will further expand during the northern hemisphere winter 2025/26 season.

Extended influenza seasons in Australia and New Zealand in 2025 due to the emergence of influenza A(H3N2) subclade K viruses.

Key Points

Abstract

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