Onset, intensity, severity and duration of influenza seasons vary each year and are driven by factors such as temperature, rainfall, humidity, circulating virus types/subtypes, population immunity through vaccination or natural infections, as well as domestic and international travel. This applies to countries in the southern as well as northern hemisphere.
In their rapid communication published in Eurosurveillance, Dapat et al. describe the influenza season 2025 in Australia and New Zealand. It was initially marked by influenza A(H1N1)pdm09 viruses and lower-level circulation of influenza B viruses. Case notifications of influenza A(H3N2) started to go up notably in August 2025 and H3N2 viruses were predominant in September in New Zealand and October−November in Australia. In contrast to influenza B type viruses, such a shift in types/subtypes or the emergence of a variant towards the end of an influenza season is rather uncommon for influenza A viruses.
The 457,906 confirmed influenza cases across Australia between January and November 2025 not only constitute a record number during an influenza season since the disease became notifiable in 2001. Stretching from May to Australian summer in November, the season was also unusually long.
New Zealand experienced a more moderate influenza season outbreaks caused by A(H3N2) K viruses, which were introduced from Australia, prolonged the season here as well.
Genomic analysis indicates Australian subclade K viruses may have originated from the United States
The first K virus sequence logged on GISAID (Global Initiative on Sharing All Influenza Data) in June 2025 was from New York, followed by detections in Wisconsin and Michigan in July 2025. This coincided with the first detections of clade K viruses in Australia. Based on this and the genomic analysis results, the authors hypothesise that A(H3N2) K viruses in Australia and New Zealand may have been imported or originated from the United States.
Overall, from A(H3N2) viruses collected and sequenced in 2025, the subclade K viruses accounted for roughly half of the Australian viruses (502/998) and more than two thirds (35/49) of the viruses in New Zealand.
Dapat et al. note that the Australian−New Zealand H3N2 K viruses tested for susceptibility to the licensed influenza antiviral drugs were susceptible to all of them. They conclude that "these antivirals may be used to ameliorate the outcomes from subclade K virus infections. These drugs are most effective if administered within 48 hours of when symptoms first appear."
According to available GISAID data until end of November 2025, A(H3N2) subclade K viruses have been detected in at least 34 countries across the world including the US, several European countries, Asia, Africa, and the Middle East.
Based on their study results, the authors consider that "given the speed and size of the outbreaks of K viruses in Australia and New Zealand and the near global spread of these viruses already, it is likely that they will further expand during the northern hemisphere winter season and persist for the remainder of 2025 and into 2026". They conclude that "countries should be prepared for increased demands on their healthcare systems if this variant predominates, as one might expect it will, based on current global trends."
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