Key points
- Subclade K, known as the ‘Super-K’ flu, has spread earlier and faster than typical seasonal influenza.
- Early evidence suggests this season’s flu vaccine is less effective at preventing infection from this strain, though it may still reduce how sick you get.
- Scientists are analysing the strain to understand its mutations, its behaviour and whether it should influence future vaccine composition.
A fast-moving influenza strain nicknamed the ‘Super-K’ flu is catching the attention of scientists and health authorities.
Subclade K, a branch of the H3N2 influenza family, has appeared much earlier than expected and is spreading quickly across Australia. Researchers are now working to understand how these mutations affect immunity, how well the current vaccine performs against it and what this might mean for the flu season ahead.
CSIRO disease prevention and detection expert Dr Daniel Layton breaks down what we know so far, and answers some of the most common questions about the ‘Super-K’ flu.
What makes subclade K of H3N2 — the so-called ‘Super-K’ flu — distinct from typical seasonal flu?
Subclade K (previously known as J.2.4.1) is gaining attention because it appears to be spreading much earlier in the flu season than usual — and it’s spreading quickly. Neither of these are good signs for an influenza strain.
It’s important to note that subclade K isn’t a brand-new kind of flu, it’s still a seasonal strain of influenza, however it has undergone a substantial number of mutations in one of its key proteins called Hemaglutinin. These changes can affect how the virus behaves and spreads.
How effective is this season’s flu vaccine against subclade K?
A big part of how our immune system prevents infection from influenza is by producing antibodies that bind to and block the Hemaglutinin protein. This protein helps the virus latch onto our cells, so when antibodies block it, infection is prevented.
Early data suggests that because subclade K has many mutations in this protein, there is a mismatch between the antibodies we develop from the flu vaccine and this strain of virus, leading to that reduced ability to prevent infection. The same is likely true of antibodies we have developed from previous infections.
However, the vaccine can still reduce how severe the illness is. In fact, data from England has shown that the current flu vaccine was 72-75 per cent effective at preventing emergency department visits in adolescents (under 18) and 32-29 per cent effective in adults.
Those percentages are somewhat typical of vaccine effectiveness in preventing emergency department presentations and hospitalisations. So, it looks like in real world numbers, the vaccine effectiveness is about on par with previous years, despite the mismatch.
What happens in the vaccine lab when a new strain is found?
It’s not uncommon for new influenza strains to be identified each year. When a particular strain shows either a high level of circulation or an increase in disease severity, that’s when alarm bells start to ring.
In the case of subclade K, the virus would go through a panel of tests to first determine its genetic sequence and subtype, to understand how similar or different it is from other strains. A next crucial step is ‘antigenic characterisation’. This means comparing the virus to antibodies created by previous vaccines to see if the new strain can slip past them and cause an infection. If it can, this is called a ‘mismatch’.
Researchers also look at the new strain’s growth rate, how well it binds to human cells and whether it shows resistance to antiviral treatments.
Why are we seeing an unusually early and large flu season, especially linked to ‘Super-K’?
When a new viral variant emerges, it is likely that we see an unusually early and large influenza season because our preexisting immunity is challenged. The number and location of the new mutations on the subclade K Hemaglutinin protein mean our antibodies struggle to attach. In turn, that means the virus can grow and spread more rapidly, leading to more influenza cases.
Will subclade K make me more sick than other seasonal influenza strains?
Even though it has been shown to be spreading very quickly and there are a lot of cases, the current best evidence suggests subclade K does not cause more severe disease per infection. That being said, historically, flu seasons dominated by H3N2 strains of influenza (subclade K is a H3N2 strain) have been linked to more severe outcomes at the population level.
I got my flu shot in winter 2025. Should I go and get the flu vaccine again now? What about if I missed last year’s vaccine?
If you have any questions about vaccines, it’s always best to talk with your GP to ensure you get the best advice for your circumstance.
If you had a flu shot in 2025, a routine extra dose now is not typically recommended, and protection is generally highest in the first 3-4 months after vaccination. The new seasonal vaccine will become available in Australia in coming months.
However, if you are in a high-risk category and didn’t get vaccinated last year, it’s worth considering. Even though the last year's flu vaccine doesn’t include subclade K, research shows it can reduce severity of symptoms. Talk with your GP about whether getting the shot now is right for you.
What could the emergence of subclade K indicate about future flu seasons?
If we see continued circulation of subclade K and in increasing high numbers, it may prompt a change in the influenza vaccine composition to include a subclade K H3N2 component. This would then allow people to develop antibodies that bind strongly to the new strain and reduce infection. These decisions are complex and will require a thorough analysis of all of the available data.
What is the process to decide what flu strains are covered each year?
Scientists around the world continually test flu samples to see which strains are spreading and how fast they’re changing.
Twice a year (once for northern hemisphere and one for southern), the World Health Organisation reviews this global data and recommends the strains most likely to be present in the upcoming flu season, so manufacturers have time to make the vaccine months in advance.
I feel pretty sick. How do I know if it’s the flu?
The flu usually comes on suddenly, with fever or chills, body aches, headache, extreme tiredness, and a cough or sore throat.
At-home COVID/flu combo rapid tests can be helpful, especially in the first few days, but a negative result doesn’t always rule out flu — so if you’re high risk or your symptoms are getting worse, seek advice and a confirmatory PCR test from your GP.