A new influenza strain, H3N2 subclade K, has emerged and is causing concern among scientists. This strain is unique enough genetically to have caught the attention of researchers, and it's already making its presence felt in places like the UK. But the big question is, will our current flu vaccines be effective against it?
The Flu Family Tree Grows a New Branch
Influenza viruses are notorious for their ability to evolve and adapt, and the World Health Organization (WHO) is constantly monitoring these changes. Each year, the WHO recommends specific flu strains to include in seasonal vaccines, aiming to stay one step ahead of these evolving viruses.
The key to this strategy lies in the small genetic changes that occur in the virus's surface proteins, particularly hemagglutinin (H). This protein is the target of vaccines, and it's the 'H' in strain names like H3N2 or H5N1.
H3N2 is a well-known seasonal flu virus that has been circulating for decades. However, the current variant, subclade K, has evolved with enough mutations to set it apart from the reference strain (subclade J.2) chosen for this season's vaccine.
Dr. Antonia Ho, a Clinical Senior Lecturer and Consultant in Infectious Diseases at the University of Glasgow, UK, explains, "The current circulating influenza A(H3N2) virus acquired seven new mutations over the summer, meaning it's quite different from the A(H3N2) strain in this year's vaccine. This could also mean the virus has changed enough to evade immunity built up from previous infections and vaccinations."
The Rapid Rise of Subclade K
Subclade K is believed to have emerged towards the end of the Southern Hemisphere's flu season. Early data from the UK and Japan indicates that it's already prevalent in most flu samples. It has also been detected in North America, but the exact proportion of samples it accounts for is unclear.
In the UK, the flu season started around five weeks earlier than usual, with increases first seen in teenagers and young adults, followed by younger children. Almost all recent cases (98%) have been influenza A, with around 84% of those being H3N2. Of these H3N2 cases, approximately 87% are subclade K.
Early flu seasons have also been observed in parts of the European Union, although overall case numbers remain relatively low. While most EU cases are H3N2, limited sequencing data makes it difficult to determine the exact proportion that belongs to subclade K.
Prof. Adam Finn at the University of Bristol, UK, suggests that an early start to the flu season could indicate that the circulating strains are spreading more easily than usual, even before the typical colder weather conditions that signal the onset of the season.
"An important possible explanation for this could be that there is less immunity than usual in the population to control the spread of flu viruses," he said.
The Severity of Subclade K
Compared to H1N1, H3N2 generally tends to cause more severe illness, especially in older adults. Dr. Ho notes, "For example, in the 2022-2023 winter season when A(H3N2) was dominant, there were 16,000 influenza-associated deaths in the UK, compared to 8,000 last winter."
An early flu season also means that vulnerable populations may not have had time to receive their flu vaccine, leaving them less protected.
Vaccine Protection Against Subclade K
Early data from the UK suggests that while the new H3N2 subclade K virus has evolved away from the strain in this year's flu vaccine, the vaccine is still providing meaningful protection. Laboratory tests in immunized ferrets, a standard model for assessing antibody recognition of circulating flu strains, indicate that their antibodies were less effective at recognizing subclade K compared to the vaccine strain.
However, antibody tests only capture one aspect of vaccine protection. Real-world evidence from the UK Health Security Agency (UKHSA) is more encouraging. The 2025/6 vaccine is currently 70-75% effective at preventing hospital attendance in children aged 2 to 17, and 30-40% effective in adults.
Flu vaccine effectiveness varies from season to season, typically falling between 30% and 60%, with higher protection often seen in younger age groups. The UKHSA considers this early data to be promising.
Dr. Jamie Lopez Bernal, Consultant Epidemiologist for Immunisation at UKHSA, said, "These results provide reassuring evidence that this season's flu vaccines currently offer important protection to children and adults, despite concerns about the new subclade. The high vaccine effectiveness in children strengthens the case for ensuring all eligible young people get vaccinated."
"Importantly, whatever strains circulate this winter, we can be confident that the vaccine will still help protect the most vulnerable from developing serious illness and being hospitalized. We strongly encourage all eligible individuals to get vaccinated against flu as soon as possible - it remains our best defense against serious illness."
And this is the part most people miss: the flu vaccine is not just about protecting yourself; it's about protecting those around you, especially the vulnerable. So, will you be getting your flu shot this season? We'd love to hear your thoughts in the comments!
