“Superflu” or same old flu? How subclade K influenza is playing out worldwide
A fast-spreading strain of influenza A (H3N2), known as subclade K, has fuelled headlines warning of a possible “superflu”. The reality is more nuanced, scientists say.
- 19 December 2025
- 6 min read
- by Linda Geddes
At a glance
- Subclade K is a newly emerged subgroup of a well-established human seasonal flu virus called H3N2 influenza A virus.
- So far, there’s no evidence from laboratory or population studies that this strain can bypass existing immunity from past influenza infections or vaccines.
- Flu vaccine effectiveness varies from season to season but typically falls between 30% and 60%. Early estimates suggest that this year’s vaccine reduces the risk of flu that’s severe enough to require medical attention by around 32-39% in adults, and by around 72-75% in children and adolescents.
A new branch of the familiar H3N2 influenza virus – known as subclade K – has rapidly become a dominant form of flu in some countries, including the UK, Japan and parts of Europe.
Because it carries a number of mutations compared with the strain used to make this year’s flu vaccine – and because it has coincided with an unusually early flu season in some countries – scientists have been watching it closely, and the term ‘superflu’ has begun to appear in headlines.
But while subclade K is genetically distinct from other types of H3N2 and other influenza viruses that are currently co-circulating, it remains closely related to the seasonal flu viruses that we’ve been exposed to many times before.
Here’s what we know about how subclade K is playing out in real life so far.
What is subclade K influenza – and how is it different from other flu strains?
Subclade K is a newly emerged subgroup of a well-established human seasonal flu virus called H3N2 influenza A virus, which has circulated for decades. Scientists first spotted this new variant in June, then noticed it expanding rapidly during the southern hemisphere’s flu season, and in several northern hemisphere countries.
“When we see a signal of a new, emerging genetic group of viruses, we do pay attention to those signals,” said Prof Nicola Lewis, Director of the World Influenza Centre at the Francis Crick Institute in London, UK, which forms part of the World Health Organization’s Global Influenza Surveillance and Response System(GISRS).
“It’s certainly true to say that this H3 new genetic clade is novel, but flu evolves consistently all the time, and our job as scientists is to track that evolution. There’s not really a signal at the moment that there’s anything particularly unusual about the way this virus is evolving.”
Has the flu season started earlier than usual everywhere?
Within Europe, subclade K was first detected in Norway, followed by the UK, where the influenza season started 4–5 weeks earlier than usual. Lewis said that, while such an early start hadn’t been seen since before the COVID-19 pandemic, it is not historically unprecedented.
This early start had also been seen in Japan, where the levels of H3N2 flu are now plateauing, and may soon start to drop, Lewis said.
Although an early start can sometimes raise questions about a virus’s ability to transmit itself, it doesn’t automatically mean a worse flu season. In the UK, where some hospitals are currently under severe pressure, the latest surveillance data suggests that the flu positivity rate is now starting to decrease.
Reassuring as this is, “flu is notoriously unpredictable and can bounce back and peak a second time in the new year and so we’re urging people to continue taking sensible steps over the festive period to help stop the spread,” said Dr Alex Allen, Consultant Epidemiologist at the UK Health Security Agency (UKHSA).
Have you read?
An early start to the flu season hasn’t been documented everywhere. Some European Union member states are only starting to see an increase in their influenza activity now, and H1 influenza viruses – another type of human seasonal flu virus – are also circulating.
In the US, influenza activity was low before Thanksgiving, but has since accelerated following holiday travel and school reopening, with the Centers for Disease Control and Prevention (CDC) reporting at least 2.9 million illnesses, 30,000 hospitalisations and 1,200 deaths so far this season. Most cases are due to influenza A (H3N2), with the K subclade now predominating.
In Australia, the flu season – which typically runs from May to October – has been unusually prolonged, with infections linked to the H3N2 subclade K variant occurring towards its tail end. While influenza notifications have been steadily declining in recent weeks, the virus continues to circulate.
Is subclade K better at evading immunity from past infection or vaccination?
One of the biggest fears with a new flu strain is that it might bypass existing immunity from past infections or vaccines.
So far, laboratory and population studies suggest this hasn’t happened. In unpublished work, Lewis and her colleagues have looked at how well people’s immune systems recognise subclade K compared with earlier flu viruses, as well as how the body responds after vaccination.
Based on these findings, “it looks like there isn’t a clear signal of a true gap in immunity that this virus has exploited,” Lewis said.
This means people’s existing defences still appear to offer meaningful protection, and vaccinated people appear to generate good antibody responses against currently circulating influenza viruses, including subclade K.
How effective is the flu vaccine against subclade K?
Early estimates published in Eurosurveillance suggest that the current flu vaccine continues to provide meaningful protection against influenza that’s severe enough to require medical attention – including infections caused by H3N2 subclade K. In children and adolescents, it is 72-75% effective, while in adults is around 32-39% effective. “This is similar to end of season vaccine effectiveness against influenza A(H3N2) in recent years in the United Kingdom, Europe and Canada,” the study’s authors said.
Crucially, researchers stressed that this does not mean the vaccine is failing against subclade K.
Early estimates published in Eurosurveillance suggest that the current flu vaccine continues to provide meaningful protection against influenza that’s severe enough to require medical attention – including infections caused by H3N2 subclade K. In children and adolescents, it is 72-75% effective, while in adults is around 32-39% effective.
The bigger concern is uptake. For instance, in the UK, while coverage among over-65s is relatively high, uptake remains “pretty low” in younger at-risk adults and health workers – groups where improved vaccination could significantly reduce transmission, hospital admissions and pressure on health services, said Prof Antonia Ho, Professor and Honorary Consultant in Infectious Diseases at the University of Glasgow in Scotland.
So, should people stop calling it a “superflu”?
“Superflu” is not a scientific term, and based on current evidence, there’s no indication that subclade K is any more dangerous than other H3N2 influenza viruses currently circulating, said Lewis.
But calling it “just flu” is also misleading. Globally, seasonal influenza is estimated to cause 3–5 million cases of severe illness and between 290,000 and 650,000 respiratory deaths each year.
H3N2-dominated seasons in particular are often challenging, especially for older adults. This subtype of influenza tends to cause more severe illness in elderly people than other strains and is associated with higher hospitalisation rates and longer hospital stays, Ho said.
That’s why public-health messaging is focused on vaccination and reducing spread. “There are simple steps we can all take to protect one another when mixing indoors,” said Allen.
“If you have flu or COVID-19 symptoms, including a high temperature, cough and feeling tired or achy, you should try to minimise contact with others, especially those more vulnerable. Washing hands regularly and ensuring indoor spaces are well ventilated helps, and we advise those who have symptoms and need to go out, to consider wearing a face covering.”
