Unraveling the 'Super-K' Flu: Unusual Variant Spreads Across Australia

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A new strain of influenza known as ‘Super-K’, or the subclade-K variant, could be responsible for an unusual spring and summer boost in flu cases, epidemiologists say.

“The unusual thing here for Australia is the timing,” WHO Collaborating Centre for Reference and Research on Influenza director Patrick Reading told SBS News.

“The not unusual thing is probably the symptoms,” he said.

Reading recalls the subclade-K variant emerged towards the end of Australia and New Zealand’s typical flu season, circulating in September rather than the winter months.

“That was responsible for elevated influenza cases throughout the spring and actually throughout the start of our summer in December as well.”

According to the Global Initiative on Sharing All Influenza Data (GISAID), the total lab confirmed cases of subclade-K in Australia so far currently sits at 1,083.

A new report by the Australian Centre for Disease Control (ACDC) stated that subclade-K’s rapid spread has the potential to put increased pressure on Australian health care services over the summer and into the 2026 influenza season.

An ACDC representative has highlighted an unusual uptick in flu cases for this time of year, noting that current figures surpass those typically observed in past seasons.

So far this year, over 4,100 cases of influenza have been confirmed by laboratories, with the true number of flu cases likely to be higher.

To put it in perspective, the weekly average of confirmed influenza cases during the first quarter of last year was approximately 3,300.

According to Bennett, while the 2025 flu vaccine does include protection against the H3N2 strain, it does not cover this particular variant.

“Every few decades, a flu virus can undergo a much larger genetic change called antigenic shift, which can lead to a pandemic, but this is not what has happened with super-K,” University of South Australia epidemiology professor Adrian Esterman said.

“Instead, super-K is best understood as a drifted version of the familiar H3N2 virus, with enough changes to allow it to spread more widely in populations with existing immunity.”

The ACDC report noted Australian subclade-K viruses may have originated in or been imported from the US, following detections in the US in June and July last year.

Reading said there’s no current evidence that the variant is more severe than a regular flu. What experts are seeing, however, is that it easily transmits.

“It appears that it’s probably going to have an ability to at least be slightly less sensitive to antibody or vaccine-mediated protection,” he said.

Deakin University epidemiology professor Catherine Bennett told SBS News: “This new subvariant of the H3N2 is that bit more immune evasive with these latest mutations in the part of the virus that allows it to invade our cells, making us more susceptible.”

Bennett said the 2025 flu vaccine included H3N2, but not this specific class.

“Flu vaccines wane in effectiveness, so people vaccinated back in April-May and only getting infected now won’t have the same benefit,” she said.

However, the 2026 flu vaccine for the southern hemisphere has already been updated to include this variant, and will likely be available around April, Bennett said.

Esterman said the flu vaccine remains worthwhile and provides important protection.

“It still significantly reduces the risk of severe disease.”

What does a typical flu season look like in Australia?

“When we get seasonal influenza every year in Australia, or anywhere, it’s a combination of three viruses,” said Reading.

There are two influenza A viruses — a H3N2 and H1N1 — and one influenza B virus.

According to the Australian Centre for Disease Control, there were 365,261 reports of the flu in 2024. Cases spiked between May and August, and drastically dropped following the winter.

Two line graphs showing the number of influenza cases in 2024 and 2025

“As we move into spring, flu generally comes down in Australia, and then it’s very low during summer,” said Reading.

Last year there were 502,493 reports of influenza, with cases clearly continuing until the end of the year following those winter months.

According to GISAID, H3N2 subclade-K is driving an estimated 90 per cent of cases in the United States.

“What we saw around September [in Australia] onwards is the H1N1s and B viruses decreased, but we saw an increase in H3N2 viruses,” Reading said.

How can I avoid catching the flu?

“People who are unwell should consider testing and should avoid mixing with other people as much as possible,” Bennett said.

“Keep homes well-ventilated, hold gatherings outdoor if possible, and wear a mask in busy public indoor spaces if susceptible to more severe respiratory infections.”

NSW Health advises getting the annual flu vaccine, as well as practising good hand hygiene — including handwashing.