Super-K H3N2 strain infects more than 2,500 Australians in early January

More than 2,500 Australians were infected in the first week of January with a highly transmissible new strain of Influenza A (H3N2) known as subclade K, or Super-K. Super-K was first identified in September by scientists at Melbourne’s Peter Doherty Institute for Infection and Immunity and, according to the institute’s deputy director Prof Ian Barr, genetic mapping suggested it originated in insignificant numbers in the US in mid-2025.

The strain has spread to more than 30 other countries, began appearing in Australia in August, and by October had taken hold; by mid-November weekly emergency department presentations for influenza-like illness spiked to more than 370 people in New South Wales. Surveillance data showed 284 infections in children aged four years and younger in the past seven days, and more than one-third of Australia’s cases in the past week were reported in New South Wales.

By mid-December the NSW Respiratory Surveillance Report recorded more than 3,000 laboratory-confirmed notifications a week, a 15% week-on-week increase during a month when flu viruses typically decline. Last year more than half a million Australians had a laboratory-certified form of flu and 1,508 people died, a 44% increase on the 2024 mortality rate.

Key Topics

Health, Super-k, Influenza A, Australia, New South Wales, Peter Doherty Institute