Australia’s 2025 Flu Season: The “Super-K” Variant and What’s Ahead for 2026
In 2025, Australia recorded its highest number of influenza-related deaths on record, surpassing COVID-19 as the leading cause of death among respiratory viruses from August onwards.
Influenza Mortality in Australia (2025)
Australia experienced its deadliest flu season this century in 2025. According to the Australian Bureau of Statistics (ABS), influenza was the underlying cause of death for 1,455 Australians—the highest number recorded in the ABS data series, surpassing previous peaks of 1,276 in 2017 and 1,072 in 2019.
The National Notifiable Disease Surveillance System (NNDSS) reported over 1,700 influenza-associated deaths in 2025, marking the deadliest flu season this century. There were approximately 500,000 laboratory-confirmed influenza cases reported across the country.
COVID-19 and RSV Mortality in 2025
For the first time since the pandemic began, influenza caused more deaths per month than COVID-19 from August 2025 to January 2026.
- COVID-19 was the underlying cause of 1,718 deaths in 2025, a significant decline from 3,908 in 2024 and 4,613 in 2023.
- Respiratory syncytial virus (RSV) was the underlying cause of 198 deaths, with an additional 392 deaths listing RSV as a contributing factor.
The H3N2 Subclade K (“Super-K”) Variant
Scientists identified a new, mutated variant of the Influenza A H3N2 strain, officially termed subclade K and colloquially referred to as “Super-K.”
This variant features multiple mutations in its hemagglutinin protein, making it antigenically distinct from the strains included in the 2025 seasonal vaccine. This results in a partial ability to evade immunity from previous infections or vaccinations.
While data does not indicate that subclade K causes more severe illness than other H3N2 strains, its high transmissibility led to a significant increase in case numbers—and therefore a higher total number of severe cases.
Subclade K was first identified in the United States in mid-2025 and has since been detected globally, including in the United Kingdom, Japan, Canada, and over 30 other countries.
Contributing Factors
Vaccine Mismatch and Vaccination Rates
The 2025 influenza vaccine was not well-matched to the late-emerging H3N2 subclade K variant.
Influenza vaccine coverage fell below targets across multiple age groups:
- Among Australians aged 65 and over, coverage was approximately 62%—below the World Health Organization target of 75%.
- Vaccination rates for children aged 6 months to 5 years were low, at 25% in 2025.
- Vaccination rates across all age groups have declined since the COVID-19 pandemic.
Population Immunity
COVID-19 has become less deadly due to population immunity from vaccination and prior infection, as well as the availability of treatments.
Meanwhile, suppression of influenza during the COVID-19 pandemic years may have led to decreased population immunity against influenza, leaving more people vulnerable.
Outlook for 2026
Early Trends (January – May 2026)
Preliminary data from the ABS showed influenza deaths in January and February 2026 were roughly double those in the same period in 2025.
However, from January 1 to May 31, 2026, the Australian Centre for Disease Control (ACDC) recorded 33,071 influenza cases—a 65% decrease compared to the same period in 2025. Reduced cases have been attributed to increased population immunity from the previous year's high activity and reduced overseas importations.
Despite the overall decrease, cases rose 15% in the last two weeks of May 2026.
2026 Vaccine Update
The 2026 seasonal influenza vaccine has been updated to include an A(H3N2) strain (A/Singapore/GP20238/2024) that is closely related to the circulating subclade K variant, offering a better match.
The 2026 vaccine also includes:
- An A/Missouri/11/2025 (H1N1)pdm09-like virus
- A B/Austria/1359417/2021 (B/Victoria lineage)-like virus
For older Australians, a higher-dose vaccine is available. A needle-free nasal spray vaccine is available for children aged 2–17.
Health authorities recommend influenza vaccination for all individuals aged six months and over, ideally in April or May for optimal protection before the peak winter season.
U.S. Influenza Activity (2024–2025 Winter)
The U.S. experienced high influenza activity driven by the H3N2 subclade K variant. The 2024–2025 season was described by the CDC as the most severe since 2017–2018.
- By late December 2025, the CDC estimated at least 7.5 million illnesses, 81,000 hospitalizations, and 3,100 deaths, including 8 pediatric deaths.
- By mid-January 2026, estimates had risen to at least 15 million infections, 180,000 hospitalizations, and 7,400 deaths.
- Vaccination coverage in the U.S. was approximately 42% for both adults and children.
Global Spread
The subclade K variant has been linked to severe flu seasons internationally:
- In Japan, the flu epidemic began earlier than usual, starting at the end of September 2025.
- The variant has been linked to severe flu seasons in the UK, Europe, and Asia.
Regional Situation: Far North Queensland
In early 2026, Far North Queensland recorded more than 300 laboratory-confirmed influenza cases, a number exceeding typical patterns for the period.
The region is experiencing an increase in international tourist arrivals, particularly from Indonesia, following the restoration of direct air routes between Bali and Cairns. The A(H3N2) subclade K strain has been identified in both Australia and Indonesia.
Recent heavy rainfall and flooding have strained hospital capacity and disrupted primary care access in remote communities.
Vaccination Recommendations
Annual influenza vaccination is recommended for all individuals aged six months and older.
High-risk groups recommended for vaccination include:
- People aged 65 and over
- Children aged 6 months to 5 years
- Pregnant women
- People with chronic medical conditions
- Aboriginal and Torres Strait Islander people
- Aged care residents
Influenza, COVID-19, and RSV vaccines can be administered during the same visit.