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. 2025 Dec;30(49):2500894.
doi: 10.2807/1560-7917.ES.2025.30.49.2500894.

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

Clyde Dapat  1   2 , Heidi Peck  1   2 , Lauren Jelley  3 , Tanya Diefenbach-Elstob  1 , Tegan Slater  3 , Saira Hussain  1 , Phillip Britton  4   5 , Allen C Cheng  6 , Tim Wood  3 , Annaleise Howard-Jones  4   5 , Yi Mo Deng  1 , Jessica E Miller  1 , Q Sue Huang  3 , Ian G Barr  1   7
Affiliations

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

Clyde Dapat et al. Euro Surveill. 2025 Dec.

Erratum in

  • Erratum for Euro Surveill. 2025;30(49).
    Eurosurveillance editorial team. Eurosurveillance editorial team. Euro Surveill. 2025 Dec;30(50):251218c. doi: 10.2807/1560-7917.ES.2025.30.50.251218c. Euro Surveill. 2025. PMID: 41414935 Free PMC article. No abstract available.

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.

Keywords: A-subtype; H3N2; Oceania; epidemic; seasonal influenza; subclade.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

The Figure concerns Australia and New Zealand, with for each country, two respective panels presented. The first panel focuses on the period from 2022 to 2025. The numbers of laboratory-confirmed influenza cases notified monthly are depicted, providing an overview of the length and intensity of each annual influenza season. In 2025, the influenza season in Australia appears to be longer, with more cases than in past seasons, while in New Zealand the season seems more moderate, but with a peak in June, followed by another peak in September. The second panel focuses on the year 2025. There, numbers of laboratory-confirmed influenza cases by notification week are stratified according to influenza type and subtype. In both countries, the dominant A(H1N1) subtype is replaced by A(H3N2) starting around mid-August.
Figure 1
Numbers of laboratory-confirmed influenza cases by month of notification in 2022−2025 and numbers of cases stratified by type and subtype, by week of notification in 2025 for (A) Australia and (B) New Zealand, respectively
The top figure panel displays a world map with pie charts representing proportions of different influenza A(H3N2) subclades based on hemagglutinin sequencing conducted in various parts of the world. Sequences used were deposited in GISAID, with collection dates from 1 June to 11 November 2025. From this panel, it appears that subclade K viruses have spread to every continent, except South America. The bottom panel shows how proportions of J.2, J.2.2, J.2.4 and K subclades have temporally evolved relative to each other at worldwide level, in Australia, and in New Zealand, respectively. Both globally and in the two countries, the proportion of subclade K strains increased in August to become dominant in September and thereafter.
Figure 2
(A) Proportionsa of haemagglutinin subclades among genetically characterised A(H3N2) strains in different parts of the world, and (B) temporal evolution of these proportions globally, in Australia (June−November), and in New Zealand, (June−October 2025)b
On the world map, migration routes of K viruses which are statistically supported by Bayesian phylogenetic analyses are depicted by lines between paired world locations. The line thicknesses are proportional to the Bayes factor. At each location, a circle is represented, with a diameter corresponding to the posterior probability of migration events from this location. The analysis reveals that subclade K viruses likely spread to Australia from the United States. Thereafter these viruses might have been exported from Australia to New Zealand. There is also support for some migration routes from the United States to Europe, West Africa, the Middle East, and South East Asia. Spread from South East Asia to South Africa may also have happened.
Figure 3
Global migration flow map of A(H3N2) subclade K viruses inferred from Bayesian phylogeographic analysis, 2025
The figure depicts 21 test isolates of influenza A(H3N2) viruses, including six subclade K viruses circulating in Australia in 2025. These isolates were tested by haemagglutination inhibition assay using antisera respectively raised against influenza A(H3N2) subclade J.2, J.2.4 and K strains. When the six subclade K viruses were tested with antisera raised against the subclade J.2 A/Croatia/10136RV/2023 vaccine strain, these antisera showed an over eightfold reactivity reduction. In contrast, when K viruses were tested with antisera raised against J.2.4 viruses, such as A/Singapore/GP20238/2024 or A/Sydney/1359/2024 these antisera had better reactivity. Moreover, all K viruses tested with an antiserum to a K virus (A/Darwin/1415/2025) reacted very well.
Figure 4
Haemagglutination inhibition titres of ferret post-infection antisera raised against various subclades of influenza A(H3N2) reacting with two isolates from 2024a and 19 Australian virus isolates generated from samples with collection dates in 2025a (n = 21 test isolates)
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