More than 2,500 Australians have rung in the new year with a highly transmissible new strain of influenza, and health authorities are on alert for what could be Australia’s worst year since tracking began 35 years ago.
Last year’s record, when more than half a million Australians contracted a laboratory-certified form of flu and 1,508 people died, was a 44% increase on the 2024 mortality rate.
The unseasonably high rate of infection for just the first week of January has been driven by a mutation of Influenza A H3N2 known as subclade K – now colloquially referred to as Super-K – first identified in September by scientists at Melbourne’s Peter Doherty Institute for Infection and Immunity.
But health authorities said there was nothing to indicate an increase in disease severity. Rather, genetic changes had allowed the strain to spread rapidly, even as vaccines continued to protect against severe illness.
Super-K has now spread to more than 30 other countries.
According to data from the National Notifiable Disease Surveillance System, 284 influenza infections in Australia over the past seven days had occurred in children aged four and younger.
Prof Ian Barr, the deputy director of the World Health Organization Collaborating Centre for Reference and Research on Influenza, told Guardian Australia that genetic mapping suggested Super-K originated in insignificant numbers in the US in mid-2025.
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The strain began appearing in Australia in August. By October it had taken hold and by mid-November weekly emergency department presentations for influenza-like illness had spiked to more than 370 people in New South Wales alone, according to data from NSW Health.
More than one-third of Australia’s cases had been reported in NSW in the past seven days.
And, as most people who contract the flu do not get tested, that figure may be a small proportion of the population infected, the state health minister, Ryan Park, said.
Yet vaccination rates have been plummeting among those most vulnerable, the Royal Australian College of General Practitioners said.
In 2025 only 25.7% of children aged six months to five years were vaccinated, the lowest since 2021, and rates for those over the age of 65 slipped to 60.5%, a five-year low.
By mid-December, the NSW Respiratory Surveillance Report was recording more than 3,000 laboratory-confirmed notifications a week – an alarming 15% week-on-week increase during a month when flu viruses typically go into hibernation.
“It does seem to be a very fit virus,” Barr said. “Normally we wouldn’t see viruses taking off that late in the season … and we’re still getting a reasonable number of cases even now in January.”
Health authorities, beginning with a national Australian Health Protection Committee (AHPC) alert on 12 December, had stressed that Super-K does not appear to be more severe than previous H3N2 strains; it was simply much more efficient at finding new hosts.
But given its unseasonable persistence, its spread could place pressure on national healthcare resources over the holiday period, the AHPC warned.
Australians travelling to the northern hemisphere are advised to get an influenza vaccine dose ahead of travel.
The Australian Centre for Disease Control said Australia’s influenza season typically peaked during winter, from June to September.
“Current case numbers for influenza are higher than usual for this time of year compared to previous seasons,” a spokesperson said.
Barr said the good news was that research coming out of the EU suggested the effectiveness of the old flu vaccine on the new Super-K virus had been “surprisingly better than expected”.
In children it was found to be 72.8% effective in keeping them out of hospital once contracting Super-K, and for 18 to 64-year-olds 66.3% effective. Like most other viruses, however, the over-65 group took a sudden dive – 31.7% effective.
The data showed the current influenza vaccination was still the best form of protection available against Super-K, Barr said.
