By Hollie SilvermanShareNewsweek is a Trust Project memberA new and potentially more severe variant of the flu, known as H3N2 subclade K, has reached the United States, prompting heightened vigilance from public health officials as the holiday season and winter travel approach.
According to the Centers for Disease Control and Prevention's (CDC) flu report, there have already been at least 650,000 illnesses, 7,400 hospitalizations, and 300 deaths from flu this season, ending the week of November 15.
Among the cases reported nationwide last week, 71.9 percent, 115 cases, were H3N2 while 26.9 percent were H1N1. The other 1.2 percent were the H5 sample specimens from the same person.
"There is basically a new variant of influenza circulating that has mutated a little bit," Caitlin River, epidemiologist at Johns Hopkins told NPR. "And that means that it's just different enough from what your body or the vaccine may recognize that it can kind of get around those protections."
Newsweek has reached out to the CDC for further comment by email Sunday.
Why It Matters
The emergence of the H3N2 subclade K variant has led experts to warn that this flu season could be particularly severe.
The strain, which has dominated outbreaks in Japan, the United Kingdom, and Canada, is showing signs of spreading rapidly in the U.S. where vaccine uptake is also lagging.
Experts stress that although the seasonal flu shot provides some protection, the new variant's genetic mutations have raised worries about reduced vaccine effectiveness, especially for vulnerable populations such as the elderly.
What To Know
The H3N2 subclade K strain first appeared in Japan and the U.K. before spreading rapidly throughout Canada, where it is now the dominant flu subtype and responsible for a rise in hospitalizations, particularly among the elderly and children.
According to the European Centre for Disease Prevention and Control, the strain has been detected on all continents since May 2025 and accounted for nearly half of H3N2 cases in the European Union (EU) by November.
Flu activity in the U.S. has increased quickly in recent weeks, with most current cases attributed to Influenza A H3N2, and about half linked to subclade K.
The CDC recently resumed full operations after a government shutdown, which lasted for more than a month, paused some surveillance activities, making precise assessment difficult for several weeks.
Recent lab data suggest a brisk uptick in positivity rates, and surveillance systems such as WastewaterSCAN confirm rising flu signals in several states, CNN reported.
The symptoms of H3N2, including its mutated subclade K, are similar to typical flu infections—fever, cough, runny nose, fatigue, muscle aches, and chills. However, reports from international outbreaks show this variant tends to cause more severe illness, particularly among the elderly and young children, sometimes resulting in higher fevers and more frequent complications.
While the current 2025–26 flu vaccine protects against H3N2, it was developed before subclade K emerged. Data from the U.K. shows that, for children, the vaccine reduces the risk of hospitalization by about 75 percent while the protection for adults, especially those over 65, is lower—around 30 percent to 40 percent, according to a report from NPR.
Despite the mismatch, experts continue to recommend vaccination, emphasizing its ability to blunt severe illness and complications.
Flu vaccination rates in the U.S. have fallen compared to previous seasons. The CDC reported 147.6 million doses distributed for the 2024–2025 season—down from 157.7 million in the season prior.
In addition to vaccination, experts urge Americans to practice preventive behaviors: wear masks in crowded indoor spaces, wash hands for at least 20 seconds, and avoid crowded gatherings if possible—especially for those who are immunocompromised or otherwise at high risk of severe disease.
...What People Are Saying
Dr. William Schaffner, infectious disease expert at Vanderbilt University Medical Center, told Local 10 News Friday: “We’re looking forward to the CDC’s laboratories and epidemiologists getting back in the swing after they’ve recovered from being away from the office for so long (during the federal government shutdown)...My fingers are crossed that they [CDC] have enough personnel … to put together this good information on a regular basis.”
Dr. Richard Webby, who studies the flu at St. Jude Children's Research Hospital in Memphis, told NPR: “The flu season might have a little bit of a punch to it this year...H3N2 viruses tend to be a little bit more problematic...With this new variant, the flu shot this year may not be as good a match as if we hadn’t seen this new variant.”
Dr. Adam Lauring, chief of infectious diseases at the University of Michigan Medical School, told CNN: "I am concerned, I guess, that we could have a big flu season this year based on what we’re seeing in other parts of the world, and particularly Europe and elsewhere."
Caitlin Rivers, epidemiologist at Johns Hopkins, told NPR: "I do have concerns that uptake of both the influenza and the annual COVID vaccine won't be as high this year as it had been in previous years...they are important for protecting against severe illness. So it's really important that people go out and get them this year."
What Happens Next?
The CDC is expected to release updated surveillance reports in the coming weeks, providing clarity on the proportion of cases caused by H3N2 subclade K.
Experts urge that it is not too late to get vaccinated, as protection can take up to two weeks to develop. If current trends abroad continue, hospitals could experience increased admissions, particularly among vulnerable populations.
Americans are advised to stay informed and to consult healthcare providers about flu risks and benefits of vaccination.
Request Reprint & LicensingSubmit CorrectionView Editorial & AI Guidelines
Add Newsweek as a preferred source on Google to see more of our trusted coverage when you search.Recommended For You
NewsMaps Show Snow Risk for Each State Over Thanksgiving 2025 Holidays3 min read
NewsMost American Monarchists Don’t Want King Trump6 min read
NewsMiss Jamaica’s Sister Issues Health Update After Shocking Stage Fall2 min read
NewsMap Shows States Where COVID Is Rising Before Thanksgiving3 min read
NewsThanksgiving Week Weather Map Shows Forecast for Each State3 min read
NewsHistoric 1902 Skyscraper Is About to Change Forever—Pictures Show How3 min readRelated Podcasts
Top Stories
NewsJasmine Crockett Mocks Marjorie Taylor Greene for Resignation5 min read
NewsMarjorie Taylor Greene’s Next Move? What Polls Tell Us4 min read
WorkHired, Managed, and Fired by AI: Stories From Those Who Have Been There6 min read
NewsNalin Haley is the Poster Boy for MAGA’s Shift Further Right6 min read
NewsDonald Trump’s Approval Rating is Eroding Among Parents3 min read
PoliticsRepublicans Are Facing a Generational Rift5 min readTrending
Higher EducationFull List of Degrees Not Classed As ‘Professional’ by Trump Admin8 min read
Winter StormWinter Storm Warning As 18 Inches of Snow To Hit5 min read
Product recallAmazon Recall: Threat-To-Life Warning Issued For Items Pulled Nationwide4 min read
SNAP benefitsSNAP Benefits To Change From December: What to Know4 min read
Social SecuritySocial Security Issues Update on 2025 Changes4 min readOpinion
OpinionConventional Wisdom: Price of Culture Edition3 min read
OpinionLatinos Catch the Fish. Why Aren’t They at the Decision-Making Table? | Opinion5 min read
OpinionWhy Is the US Punishing Legal Immigrants? | Opinion5 min read
OpinionWhat Is an American?6 min read
OpinionConventional Wisdom: MAGA Infighting Edition2 min read
