Flu season comes around every year, but the end of 2025 promises to be a little more unpredictable. Watching outbreaks unfold in Europe and Australia, which is usually a good harbinger of what's happening in North America, experts are surprised. A recently mutated strain of influenza known as subclade K is currently circulating widely, and people are understandably wondering if they'll still be covered by the flu shot this year.
Fortunately, early data suggests that the vaccines are still doing their job. But the season is starting early, vaccination rates are lagging behind previous years and health workers are bracing for a busier winter.
New influenza strain: subclade K
According to the Centers for Disease Control and Prevention (CDC), the majority of current flu cases involve the H3N2 type A strain. About half of them belong to subclade K, the same variant that caused Australia's particularly severe 2025 flu season.
When scientists choose which strains will be included in the annual flu shotthey rely on what circulates in February. Subclade K was not a major player at this point, so this year's vaccine targets a slightly different member of the H3N2 family. And although they are related, the K subclade has accumulated enough genetic changes to attract the attention of scientists.
Like everyone else influenza virusesH3N2 is constantly evolving. The World Health Organization is closely monitoring these changes, as even small changes can change how well a vaccine matches the circulating strains. The most important changes occur in hemagglutinin (H), the protein vaccines teach your immune system to recognize the virus. This is the “H” in names like H3N2 or H5N1.
Subclade K discovered seven new mutations over the summer, making it markedly different from the strain chosen for this year's jab, according to a preliminary report from the UK Health Safety Authority (UKHSA). Although it is unclear whether the variant makes individual infections more severe, H3N2 in general tends to affect older people more severely than H1N1. Experts say that if subclade K spreads quickly, more infections could itself lead to more hospitalizations. CNN.
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Early start of flu season makes a difference
Influenza activity is still considered low nationally, but numbers are increasing, especially among children. The timing is similar to other early-start years, including last season, according to the CDC.
Subclade K appears to have emerged late in the season in the Southern Hemisphere, and early reports from Britain and Japan show its dominance in most influenza samples. Adam Finn, from the University of Bristol, told the GAVI vaccine alliance that such an early surge usually means circulating strains are spreading more easily than usual, even before winter weather gives them a boost.
Additionally, when flu season begins, many high-risk people have not yet been vaccinated, meaning greater vulnerability when cases begin to rise, he said.
Does the current vaccine work?
Although subclade K is different from the strain included in the current vaccineHealth care workers expect protection to remain strong.
Real-world data from the UK shows that this year's flu jab is 70 to 75 percent effective in preventing vaccinated children (ages 2 to 17) from going to hospital, and 30 to 40 percent effective in adults. These figures, however, are obtained early in the season, before immunity has had time to develop, and are taken from a preprint of the UKHSA study.
Of greater concern is that fewer people are getting vaccinated. Between August 2025 and the end of October 2025, 2 million fewer flu shots were administered at U.S. pharmacies than during the same period last year, according to prescription data company IQVIA. A similar decline was observed in Australia, and where subclade K dominated, the Australian Department of Health reported a record 443,000 cases of influenza.
Taken together, the combination of the new variant, the start of the season and lower vaccination rates suggests this winter may be more difficult than usual, but the flu shot still provides meaningful and real protection.
This article does not contain medical advice and should be used for informational purposes only.
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