Health experts say a new influenza strain known as "Super-K", or the H3N2 subclade-K variant, is driving an unexpected rise in flu cases across Australia during spring and summer - well outside the country's typical flu season. Epidemiologists say the most striking feature of the outbreak is not the severity of illness, but its timing.
Health experts say a new influenza strain known as "Super-K", or the H3N2 subclade-K variant, is driving an unexpected rise in flu cases across Australia during spring and summer - well outside the country's typical flu season.
Epidemiologists say the most striking feature of the outbreak is not the severity of illness, but its timing.
"The unusual thing here for Australia is when it's happening," said Professor Patrick Reading, director of the WHO Collaborating Centre for Reference and Research on Influenza. "What's not unusual are the symptoms themselves."
The subclade-K variant first appeared late in the traditional flu season, circulating from September and continuing through spring and into early summer.
"That led to elevated influenza activity right through to December," Professor Reading said.
Data from the Global Initiative on Sharing All Influenza Data (GISAID) shows more than 1,080 laboratory-confirmed cases of subclade-K have been recorded in Australia so far.
A recent report from the Australian Centre for Disease Control (ACDC) warned the variant's rapid spread could place additional pressure on healthcare services through summer and into the 2026 influenza season.
"Influenza case numbers are currently higher than we would usually expect for this time of year," an ACDC spokesperson said.
More than 4,100 laboratory-confirmed influenza cases have already been reported nationally this year, with health authorities noting the true number is likely significantly higher. By comparison, early 2025 saw weekly averages closer to 3,300 cases.
Experts stress the variant does not represent a major genetic shift that would signal pandemic risk.
"Every few decades we see a major antigenic shift that can lead to a pandemic - this is not one of those events," said University of South Australia epidemiologist Professor Adrian Esterman.
"Super-K is better understood as a drifted version of the familiar H3N2 virus, with enough changes to spread more efficiently in populations that already have some immunity."
The ACDC believes Australian cases may have originated from, or been imported via, the United States, where detections were recorded in mid-2025.
While there is no evidence the strain causes more severe illness than seasonal flu, experts say it appears to spread more easily.
"There's some indication it may be slightly less sensitive to antibodies generated by past infection or vaccination," Professor Reading said.
Deakin University epidemiologist Professor Catherine Bennett said recent mutations allow the virus to better evade immunity.
"This subvariant is a bit more immune-evasive, particularly in the part of the virus that helps it enter our cells, which makes people more susceptible," she said.
Australia's 2025 flu vaccine includes protection against H3N2 strains, but not this specific subclade. Experts say waning immunity may also be contributing to the current surge.
"People vaccinated in April or May won't have the same level of protection many months later," Professor Bennett said.
The good news, she added, is that the 2026 southern hemisphere flu vaccine has already been updated to include the subclade-K variant and is expected to be available around April.
Professor Esterman said vaccination remains crucial.
"It still provides strong protection against severe illness and hospitalisation," he said.
Australia's flu season typically peaks in winter, driven by two influenza A strains - H3N2 and H1N1 - and one influenza B virus.
In 2024, more than 365,000 flu cases were reported nationally, with infections peaking between May and August before dropping sharply.
"Normally, flu activity declines in spring and is very low in summer," Professor Reading said.
However, last year saw influenza cases persist well into the warmer months, with more than 500,000 cases recorded overall.
International data suggests the subclade-K variant is now dominant in the United States, accounting for an estimated 90 per cent of cases.
"What we saw from September onwards in Australia was a decline in H1N1 and influenza B, but a clear rise in H3N2 viruses," Professor Reading said.
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