In 2025 Australia recorded its biggest influenza season on record (since 2001) based on NNDSS laboratory confirmed cases. The season was mostly dominated by influenza A(H1N1)pdm09 viruses, with some circulation of influenza B until the emergence of a new A(H3N2) variant, now known as subclade K later in the season. These viruses were first detected in New York in June 2025 and in other US states in July 2025 along with their first detections in Australia. From August onwards influenza A(H3N2) cases grew rapidly with subclade K viruses causing a significant second yearly influenza peak from October-December, leading to a prolonged influenza season of approximately 33 weeks. While the transmission of the K viruses appeared to be very efficient, rapidly spreading to all states in Australia, it did not appear to be more severe than previously circulating A(H3N2) viruses. Hospitalisations and deaths did occur as a result of infections with K viruses, but these were not exceptional. Phylogeographic mapping of the K viruses showed that after the viruses arrived in Australia they spread further to several countries including New Zealand, Singapore, Fiji, New Caledonia and the Philippines.
As the K viruses were antigenically and genetically distinct from previously circulating A(H3N2) viruses (including the A(H3N2) component of the Australian 2025 influenza vaccine), it was important to determine the vaccine effectiveness (VE) of the 2025 vaccine against the newly emerging K viruses. To do this we used a test negative design case-control method to calculate the VE against medical attendance through the ASPREN general practitioners’ network as well as the VE against hospitalization through the FluCAN/PAEDS network. Somewhat surprisingly, the VE was similar to previous seasons and other subtypes. The VE for preventing medical attendance for A(H3N2) K viruses for all age groups was 48% (95% CI; 6, 71) and for children (<18 years) 72% (95% CI; 10, 91) and for adults (18-64 years) 43% (95% CI; -41, 77); estimates for elderly (65+ years) had insufficient data. While the VE for prevention against hospitalisation for all age groups for A(H3N2) K viruses was 31% (95% CI; 7, 50) and for children (<18 years) 41% (95% CI; 8, 62) and for elderly (65+ years) 55% (95% CI; 17, 76); estimates for adults (18-64 years) had insufficient data. These data highlight the value in being vaccinated against influenza even in the face of a newly emerging, distinct virus.