As the United States heads into flu season, an unexpected influenza strain threatens to make the coming months especially harsh.
Canada, the United Kingdom, and Japan have all recently reported an uptick in flu cases, with early data suggesting that subclade K — a variant of influenza A (H3N2) — is fueling the spread, according to the Global Center for Health Security.
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This could spell trouble for the U.S. if subclade K comes here.
Trouble Abroad May Signal Trouble Ahead for the U.S.
A study published this month in the BMJ predicted that the 2025–2026 flu season in the United Kingdom, which got off to an early start, could be one of the worst ever. The dominant H3N2 influenza strain mutated seven times over the summer, making it more severe than normal, according to the report.
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In Japan, health officials have declared the flu an epidemic as an uncommonly high number of cases have been reported for this time of year.
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“Because influenza viruses don’t need a passport, I think it’s only a matter of time before this strain shows up in the United States,” says William Schaffner, MD, an infectious disease specialist and a professor of preventive medicine and health policy at the Vanderbilt University School of Medicine in Nashville, Tennessee. “It may even be here now, but because of the government’s recent shutdown, we haven’t heard much information about flu now for weeks from the CDC [Centers for Disease Control and Prevention].”
Even though these signs point to a potentially bad flu season, Dr. Schaffner urges the public not to panic. “We’ll have to see how this all plays out; it’s a little bit too early to tell,” he says. “Flu is fickle. It almost always defies predictions in one way or another.”
As the CDC gets back up to speed in the days ahead, more data regarding flu trends in the United States should become available, according to Schaffner.
Initial post-government-shutdown information has shown low influenza activity in the United States, but it has begun to inch back up.
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A ‘Mismatched’ Vaccine May Leave Americans More Vulnerable
Recently, Canadian researchers published a paper warning that this year’s flu vaccine formula is a “mismatch” for what appears to be the leading strain, subclade K.
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Because this year’s shot was not designed to mainly target subclade K, the variant may better elude vaccine protection, driving infection rates up.
Flu vaccine formulas for the Northern Hemisphere are updated each year based on variants that circulated during the flu season that just passed in the Southern Hemisphere.
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This season’s shot includes components to protect against influenza A (H1N1) and influenza A (H3N2), as well as a type of influenza B.
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Because the H3N2 variant subclade K took off at the tail end of the Southern Hemisphere flu season, vaccine makers in the Northern Hemisphere did not target it, as reported by the Center for Infectious Disease Research and Policy Research.
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Minimal early data from public health laboratories nationwide show that about 63 percent of influenza A samples tested have been H3N2 and 37 percent have been H1N1, but the proportion that may be subclade K is not yet clear.
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In a statement, Antonia Ho, PhD, a clinical senior lecturer and consultant in infectious diseases at the University of Glasgow, warned that of the two seasonal influenza A viruses, “the current dominant circulating virus [A/H3N2] tends to cause more severe illness than A/H1N1, particularly in older adults.”
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Vaccination Still Provides Significant Protection
Even though this year’s flu vaccine may be somewhat of a mismatch for subclade K, infectious disease experts expect it to substantially heighten defenses against infection and especially help prevent severe illness, hospitalizations, and deaths.
“There are different strains of influenza every year, so unfortunately we don’t have a universal vaccine that you take only once,” says Aaron Glatt, MD, the chief of infectious diseases at Mount Sinai South Nassau hospital in Oceanside, New York, and a spokesperson for the Infectious Diseases Society of America. “The current vaccine is based on what went on in the Southern Hemisphere in their winter to guess what will be going on here in the Northern Hemisphere, so it’s not a perfect game.
“There are always matches and mismatches,” he says, “but those who are vaccinated should be more protected against this particular strain, subclade K. And even if they do get the flu, they are far more likely to have a milder episode than if they didn’t get the vaccine.”
Preliminary research in the United Kingdom bears this out, indicating that vaccine effectiveness against hospitalization at this point early in the season is within a typical range of 70 to 75 percent in children and 30 to 40 percent in adults.
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The CDC recommends annual flu vaccination for everyone ages 6 months and older, and stresses that vaccination is especially important for groups who are more vulnerable to severe illness, including older adults, pregnant individuals, people with chronic medical conditions, those with weakened immune systems, and young children.
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Nearly 9 out of 10 children who died from the flu in the 2024–25 season were not fully vaccinated.
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Even people with strong immune systems can benefit from the vaccine.
“This virus can make even very healthy people very sick,” says Schaffner. “We emphasize that all people get vaccinated — not just the most vulnerable but also their caregivers and their family members. Nobody wants to be a dreaded spreader.”
With peak flu season arriving in January or February, it’s not too late to get your shot, but Schaffner urges people not to delay it — the immune system takes about 10 days to two weeks to fully activate its protection against the virus.
