Flu season isn’t in full swing yet in the United States, but a mutated strain of the virus that has caused severe outbreaks abroad is sparking concern about the winter ahead.
A new version of influenza A H3N2 emerged earlier this year after picking up a number of mutations, which may improve its ability to escape immunity from vaccines, experts say.
The mutated H3N2 strain, called “subclade K,” has spread rapidly and 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, tells TODAY.com.
The subclade K strain is also sweeping the United Kingdom, which has seen a wave of severe infections earlier than expected. Health officials have warned that the U.K. is facing one of its worst winters yet as the mutated H3N2 strain spreads, per the BMJ.
Recently, it has been circulating in Canada, prompting scientists to warn about a mismatch between the H3N2 in the current flu vaccine and subclade K, which is projected to become the predominant strain worldwide.
“Knowing that there’s a new mutated strain out there and H3N2 generally causes more severe disease is concerning,” says Hopkins.
In the U.S., flu season typically peaks between December and February, and activity can persist into May, according to the U.S. Centers for Disease Control and Prevention.
H3N2 subclade K has been detected in the U.S., but a lapse in flu data published by the CDC during the government shutdown has made it difficult to get a clear picture about what’s happening in real time, experts say.
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 in humans 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.
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 know how flu season will unfold in the U.S., but experts say a lag in the data is making it harder to track flu activity. During the 44-day government shutdown, the CDC did not publish any new respiratory illness data.
According to the CDC’s latest “FluView” surveillance report for the week ending Nov. 8, flu activity remains low nationally, but cases are increasing, especially among children. Test positivity, outpatient visits and hospitalizations are also rising.
Data from WastewaterSCAN, which monitors diseases through municipal wastewater systems, show that concentrations of flu A are increasing.
“We’re not necessarily seeing an early season,” says Hopkins. However, cases are expected to rise in the coming weeks, especially after holiday gatherings. The emergence of H3N2 subclade K has some experts fearing a harsh season ahead.
The 2024-2025 flu season in the U.S. was classified as a “a high severity” season, according to the CDC.
“We don’t generally see two severe flu seasons on top of each other, but there’s no rules in this game, so it certainly is possible that we’re going to have another severe season,” says Hopkins.
Where is subclade K spreading in the U.S.?
The majority of the flu cases reported so far are H3N2. However, the CDC did not publish details about specific subclades circulating in its latest FluView report.
Given that subclade K is spiking in Canada and viruses don’t respect borders, it has likely been in the U.S. for some time, experts say.
“We’re flying a little bit blind,” Dr. Natalie Azar, NBC News medical contributor, told TODAY in a Nov. 13 segment. A rapid flu test wouldn’t be able to tell you which subclade of H3N2 or even which strain of flu A you’re infected with, she adds.
Instead, scientists use genomic sequencing to track new variants.
Sequencing data from the Global Initiative on Sharing All Influenza Data (GISAID) database show the subclade K strain is spreading in North America, but it’s unclear what percentage of cases it’s fueling, according to a study published Oct. 31.
According to Pekosz, his lab just got its first batch of flu virus sequences of the season from the Baltimore, Maryland, area — 24 out of 27 were H3N2 subclade K, he adds.
It’s unclear if data on the spread of H3N2 subclade K in the U.S. will be published by the CDC.
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.
The flu shot doesn’t always prevent you from getting infected, but it does protect against severe symptoms, hospitalizations 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. Getting vaccinated now will ensure you’re protected during the holidays.
“The fact that there is a variant circulating that’s a little bit different from the vaccine shouldn’t in any way minimize people’s desire or need to get the vaccine,” says Pekosz.
New flu strain 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, isolate for 24 hours or until you’re fever-free.
- 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.
This story first appeared on TODAY.com. More from TODAY:
- This Teacher Works in a High-Poverty School. Her Class Is the Only One Passing State Exams
- Entrepreneur Emma Grede Says Her Business Success Stems From Her Dyslexia Diagnosis
- The No. 1 Most Important Alzheimer’s Disease Symptom a Doctor Monitors in Herself
This story uses functionality that may not work in our app. Click here to open the story in your web browser.
Want more insights? Join Working Title - our career elevating newsletter and get the future of work delivered weekly.
