A mutated strain of influenza that caused severe outbreaks abroad is spreading rapidly across the United States, and with holidays in full swing, it’s prime time for the virus to surge.
As NBC’s Today Show reports, experts are already warning that this flu season seems more severe and earlier than usual, especially among children who haven’t gotten their flu shots. Data from the U.S. Centers for Disease Control and Prevention (CDC) show cases are rising, along with hospitalizations and deaths.
The surge in illness appears to be driven by a new version of influenza A H3N2, which emerged after picking up a number of mutations that may improve the virus’s ability to escape prior immunity from vaccines, experts say. The highly contagious H3N2 strain, called “subclade K,” has already gained dominance in several countries in the Northern hemisphere.
It caused a surge in infections in Japan, which declared an influenza epidemic after experiencing an unusually early and harsh flu season, Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, said in an interview with TODAY.com.
Subclade K is also sweeping the United Kingdom, which is facing one of its toughest winters yet as the mutated strain spreads, officials cautioned in the British Medical Journal (BMJ). And the variant has been circulating in Canada, prompting scientists to warn about a mismatch between the H3N2 in the current 2025-2026 flu vaccine and the subclade K strain.
In the U.S., H3N2 subclade K is already circulating in most states, according to the Global Initiative on Sharing All Influenza Data (GISAID) database. “Knowing that there’s a new mutated strain out there and H3N2 generally causes more severe disease is concerning,” says Hopkins.
So far this season, the CDC estimates that there have been 4.6 million cases, 49,000 hospitalizations and 1,900 deaths from the flu in the U.S., but the real numbers could be even higher, experts say. Flu has cause some schools to close before the winter break, and some hospitals are increasing visitor restrictions.
Here’s what to know about H3N2 subclade K strain, symptoms to watch for, and what to expect this flu season.
New Mutated Flu Strain: H3N2 ‘Subclade K’
There are four types of the influenza virus (A, B, C and D) — influenza A and B cause seasonal epidemics every winter, per the CDC. The predominant strains that circulate in the U.S. are H1N1 and H3N2 (both influenza A) and influenza B. These are the three strains included in this year’s influenza vaccine, Andrew Pekosz, Ph.D., a virologist at the Johns Hopkins Bloomberg School of Public Health, tells TODAY.com.
However, the flu mutates rapidly, says Pekosz, and these mutations can give the virus an advantage. This is exactly what H3N2 did earlier this year. “One strong type of H3N2 emerged that looks like it has mutations that will evade the immunity that the vaccine gives,” says Pekosz.
The subclade K H3N2 strain has seven new mutations.
It was first detected in Europe in June, after scientists selected the specific strains to be included in this year’s flu shot, says Hopkins. “We think vaccine immunity won’t recognize this subclade K H3N2 virus as well,” Pekosz adds. These genetic changes caused subclade K to “drift” from the H3N2 strain in the 2025-2026 flu vaccine, the CDC says.
What also concerns experts is that the emergence of subclade K H3N2 coincided with early, severe flu seasons in several countries where the strain is dominant — namely, Japan and the U.K. H3N2 is generally considered more severe across all ages, but especially among the elderly and young children, says Pekosz. “When we see early H3N2 activity, I think that’s particularly worrisome,” he adds.
Will the New Flu Strain Make for a More Severe Flu Season?
It’s too soon to predict how the 2025-2026 flu season will unfold in the U.S. but experts are already worried it will be severe due to the emergence of H3N2 subclade K, NBC News reports.
According to the CDC’s latest “FluView” surveillance report for the week ending Dec. 13, flu activity is increasing in most parts of the country. The states with the highest influenza-like illness activity are Colorado, Louisiana, New Jersey, and New York, where cases are five times greater than this point last year.
Flu test positivity rates, outpatient doctor’s visits and flu-related hospitalizations are all increasing. Two more pediatric flu-associated deaths were reported in the U.S. last week, bringing the total for the season to three, per the CDC. The latest data from WastewaterSCAN, which monitors diseases through municipal wastewater systems, show that concentrations of flu A are “high” nationally. Since November, flu concentrations have increased 260%, a spokesperson tells TODAY.com.
Per Walgreens data, purchases of over-the-counter flu tests have increased 61% over the past two weeks, and supply-chain experts told NBC News that some pharmacies are struggling to meet demand for Tamiflu, an antiviral medication. And cases are expected to keep surging after holiday gatherings.
The 2024-2025 flu season in the U.S. was classified as a “a high severity” season, according to the CDC. And while “we don’t generally see two severe flu seasons on top of each other … there’s no rules in this game, so it certainly is possible that we’re going to have another severe season,” says Hopkins.
Is the Flu Shot Effective Against the New Strain?
Scientists choose which flu strains to include in the annual vaccine based on what is circulating globally earlier in the year, says Hopkins. The 2025-2026 flu vaccine contains H1N1, H3N2 and influenza B. The mutated subclade K strain emerged too late for scientists to update this season’s vaccine. Although it’s not a perfect match to the H3N2 strain they selected, the vaccine will still offer protection throughout flu season, the experts emphasize.
While the flu shot doesn’t always prevent you from getting infected, it does protect against severe symptoms, hospitalization and deaths. “The goal of vaccines is to reduce the severity of illness, and reduce the disease impact on our population,” says Hopkins. Additionally, this year’s flu shot is expected to be a good match to the H1N1 and influenza B strains circulating, says Hopkins. “You’ll get the protection against two other strains of flu, even if H3N2 isn’t optimal,” Pekosz adds.
The flu shot is especially important for high-risk individuals, including people over 65, young children and people who are immunocompromised or have underlying conditions. “It’s not too late to get the flu vaccine,” says Pekosz. It takes about two weeks for the body to build up immunity.
Flu 2025 Symptoms
The symptoms of the mutated H3N2 flu strain appear to be similar to those caused by the usual seasonal influenza A strains, the experts note. These include:
- Fever
- Chills
- Body aches
- Headache
- Extreme fatigue
- Congestion or runny nose
- Coughing
Flu symptoms tend to start very suddenly, says Hopkins. It’s that “hit-by-a-truck” feeling. The symptoms of flu can look similar to those caused by COVID-19 and other viruses, which is why testing is important. There are now 3-in-1 rapid home tests for influenza A, influenza B and COVID-19 — the experts recommend stocking up if you can.
If you test positive for the flu, you can seek treatment with antivirals, which can reduce symptom severity, Hopkins says.
In most cases, the flu will resolve on its own without treatment after five to seven days. If your symptoms are severe or you have a high fever, signs of dehydration, or trouble breathing, contact your health care provider right away. “The flu is not just a cold. It can be quite severe,” says Hopkins.
How to Protect Yourself From the Flu
During respiratory virus season, people can take simple actions to protect themselves and others, the experts note:
- Stay up to date on vaccines.
- Test if you have symptoms.
- If positive, stay home until you’re fever-free for 24 hours without medication and your symptoms are improving; if you don’t have a fever, stay home for five days after symptom onset.
- Avoid sick people.
- Wear a mask in crowded, indoor spaces.
- Wash your hands frequently.
- Cover coughs and sneezes.
“These small things can make a big difference if we all sort of pitch in and think about our community,” says Pekosz.
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