What an unusual season tells us about the year ahead
Influenza is a virus that evolves continuously, but the 2025 season in Australia followed patterns that differed from what is typically observed. Influenza activity remained elevated longer than usual, extending well beyond winter and was marked by shifts in the dominant circulating strains. Towards the end of winter, a fast-spreading H3N2 variant emerged and, in August, was a leading cause of respiratory-related deaths in Australia.
The rapidly spreading virus was a newly identified variant of H3N2 viruses known as subclade K. First detected in Australia and New Zealand in August, it has since circulated widely and, at the time of publishing this article, has been identified in over 30 countries worldwide.
Scientists emphasise that subclade K is not an entirely new virus but a variant that has evolved from H3N2 strains that circulated earlier this year. Subclade K viruses have ‘drifted’ to accumulate mutations in one of the surface spikes of the virus, known as the hemagglutinin, such that these viruses may be less well recognised by immune responses generated from previous infection or vaccination.
The Royal Melbourne Hospital's Professor Patrick Reading, Director of the WHO Collaborating Centre for Reference and Research on Influenza (WHO CCRRI) at the Doherty Institute, said Australia's early detection of subclade K reflected the strength of national surveillance systems.
“Australia was among the first countries in the world to spot this new subgroup emerging,” Professor Reading said.
“As the season progressed, it became clearer that subclade K viruses are noticeably different to H3N2 viruses included in the vaccine used in Australia in 2025, widening the gap between the circulating strain and the vaccine’s target.
“However, our current vaccines still provide meaningful protection against influenza,” he added.
The increasing diversity in circulating influenza viruses was central to discussions at the WHO influenza vaccine composition meeting held in Japan in September, where experts updated two of the three vaccine components for the 2026 Southern Hemisphere season, including the H3N2 component. The decision reflected the pace of viral evolution and the responsiveness of the global influenza surveillance system.
“It is always a bit of a race to update the vaccine to give it the best chance of inducing immune responses that provide effective protection during our influenza season next year,” Professor Reading said.
This decision drew on global surveillance data and on work led by the Doherty Institute’s WHO CCRRI, which contributed the newly selected prototype H3N2 component of the vaccine.
These and other developments were discussed in detail at the 16th Australian Influenza Symposium, hosted at the Doherty Institute last month. Experts discussed the latest developments in surveillance, genomics, antiviral therapeutics and vaccine science with the aim of reducing the impact of influenza and other respiratory diseases.
National analyses presented by Jenna Hassall and Lauren Kutzner from the Australian Centre for Disease Control (CDC) showed that in 2025, influenza activity remained elevated well into spring, with more than 427,000 influenza notifications to the National Notifiable Diseases Surveillance System (NNDSS), about 21 per cent more than in 2024.
Influenza notifications in 2025 are likely to be the highest number of notifications to the NNDSS since influenza became nationally notifiable in 2001.
The CDC report recorded 977 deaths due to influenza in the first nine months of 2025. In the same period, more than 1,400 people died from COVID-19 – around half the number reported over the same timeframe in 2024 and significantly fewer than in earlier years. Despite this decline, COVID-19 remained one of the leading causes of death from acute respiratory infections between 2023 and 2025.
For Harry Stannard, a medical scientist and PhD candidate at the Doherty Institute who attended the symposium, the national data underscored the real-world impact of respiratory viruses.
“These numbers represent real families and real losses,” Stannard said. “They reinforce the importance of preventing these viruses from spreading and motivate ongoing research into influenza therapeutics.”
In 2025, COVID-19 vaccine coverage fell to 11 per cent, according to the Australian Immunisation Register. Influenza vaccine coverage reached almost 31 per cent, quite similar to 2024, but this was lower than in both 2023 and 2022, as reported in the Australian Respiratory Surveillance Report.
Despite lower coverage, influenza vaccination continued to provide meaningful protection, with Australian studies from the Influenza Complications Alert Network (FluCAN) and The Australian Sentinel Practices Research Network (ASPREN) surveillance systems showing that vaccinated people in 2025 were roughly 56 per cent less likely to attend general practice with influenza and around 49 per cent less likely to be hospitalised, compared with unvaccinated people, consistent with protection seen in previous years.
Australia’s experience in 2025 reflected global patterns observed through the WHO Global Influenza Surveillance and Response System (GISRS). Several Northern Hemisphere countries reported earlier-than-usual influenza activity, while parts of the Southern Hemisphere experienced prolonged seasons extending into spring.
Beyond seasonal influenza, the Symposium also considered the wider landscape of respiratory virus disease. International speakers discussed avian H5N1 influenza ‘spillover’ infections affecting dairy cows and other mammals, as well as occasional human cases. Other talks discussed the first known human case of an avian H5N5 infection, swine-origin influenza infections linked to state fairs in the US and new cohort studies examining long term immune responses to influenza and other respiratory viruses.
As experts considered what the 2025 season tells us for the year ahead, one message remained consistent. Ongoing preparedness supported by continuous surveillance, strong laboratory networks and clear public communication will be essential as Australia moves into another influenza season.
“Even when influenza viruses drift, vaccines continue to reduce severe disease complications and death,” Stannard reflected. “Respiratory viruses evolve quickly but our ability to track and respond to them is also advancing.”
The Doherty Institute and its national and international partners will continue advancing real time genomic surveillance, strengthening global data sharing, refining vaccine strain selection and supporting communities with evidence-based communication.
“This season showed that when influenza viruses change, public health systems must adapt just as quickly,” said Professor Reading.
“Continued investment in surveillance, vaccines and new antiviral treatments remain essential to the reduce severe illness and save lives as we head into future influenza seasons.”
More reading on the K virus emergence in Australia and New Zealand
Peer review: Dapat C, et al. Extended influenza seasons in Australia and New Zealand in 2025 due to the emergence of influenza A(H3N2) subclade K viruses. Eurosurveillance (2025). DOI: https://doi.org/10.2807/1560-7917.ES.2025.30.49.2500894
More updates and news from the Doherty Institute
Public Health
