Reports of a “super flu” abound just in time for the holidays. Cases of respiratory illnesses are on the increase, with the Centers for Disease Control and Prevention (CDC) reporting 11 hospitalizations per 100,000 people. Meanwhile, across the Atlantic Ocean, according to the U.K.’s Health Secretary West Streeting in The Standard, the National Health Service is facing the worst pressure since COVID-19.
Much of this has been attributed to the rise of a new variant: subclade K. Subclade K surfaced in the summer and has since become the dominant form of flu in several countries, including Japan, the U.K. and several U.S. states.
What Do We Know About Subclade K?
Influenza viruses can broadly be split into four types – A, B, C, and D, with A and B viruses responsible for the seasonal epidemics experienced each year. Subclade K is an influenza A virus. Specifically, it is a version of the H3N2 subtype.
It is natural for influenza viruses to evolve over time as small mutations create slight changes in the virus’ genetic makeup, a process known as antigenic drift. Over the last year, a particular version of H3N2 has acquired several new mutations that make it genetically distinct from previous versions of the virus, resulting in the emergence of a new subclade.
According to the World Health Organization (WHO) in a news release, “These subclade K viruses have several changes from related A(H3N2) viruses. Current epidemiological data do not indicate an increase in disease severity, although this subclade marks a notable evolution in influenza A(H3N2) viruses.”
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Why Are Experts Concerned?
While there is no indication that subclade K is any more severe than other H3N2 type viruses, experts have raised a couple of concerns – one that relates to vaccines, and another connected to its subtype.
Because influenza viruses are constantly changing, the flu shot must be updated every year. But the timing of subclade K has been particularly challenging. The vaccine was designed before subclade K emerged, leading to concerns that vaccines might not be a “good match” and immunization might not necessarily lead to better protection against this strain of the virus, according to the CDC.
The evidence so far suggests this is not necessarily the case. The efficacy of the flu shot varies year on year, tending towards 30 percent and 60 percent, according to Vaccine Alliance Gavi. The WHO notes the vaccine’s effectiveness against the disease is unknown but early evidence suggests it does reduce the likelihood of being admitted to the hospital.
Data collected in the U.K. from Eurosurveillance suggests the vaccine reduces the chances of an emergency room visit by 72 percent to 75 percent in children. The same data suggests the vaccine is less effective in adults but still lowers emergency room visits by 32 percent to 39 percent.
A second concern is that subclade K is a H3N2 virus. Generally, these viruses can lead to more severe illnesses than those of other flu viruses, resulting in a particularly tough flu season, with the youngest and oldest most at risk.
The Impact of Subclade K
While some countries – including the U.K. and Japan – have reported an unusually early flu season, the CDC reports the timing of flu activity in the U.S. is comparable to previous years.
But much like other countries across the Northern Hemisphere, it does appear subclade K is dominating. Recent data from the CDC’s FluView report reveals influenza A accounted for 98 percent of flu cases reported by public health laboratories, of which 89.9 percent were from the H3N2 subgroup. Nine in 10 samples of H3N2 studied by the CDC are from subclade K.
This article is not offering medical advice and should be used for informational purposes only.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from JAMA: A New H2N2 Influenza Strain Is Raising Concerns About This Flu Season
- This article references information from the World Health Organization: Seasonal influenza - Global situation
- This article references information from Eurosurveillance: Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K
- This article references information from the Centers for Disease Control and Prevention: Weekly US Influenza Surveillance Report: Key Updates for Week 50, ending December 13, 2025
- This article references information from Vaccine Alliance Gavi: “Superflu” or same old flu? How subclade K influenza is playing out worldwide
- This article references information from an article in The Standard: 'Worst crisis since Covid': Super flu tearing through London as Streeting urges doctors to call off strike
- This article references information from the Centers for Disease Control and Prevention: Key Facts About Seasonal Flu Vaccine
