Each fall and winter brings a new flu season and with it new iterations of the influenza virus. This year, a new flu strain is raising concerns that the 2025-2026 flu season may be particularly severe.
A highly mutated form of the influenza A H3N2 virus, known as subclade K, has rapidly become the most prevalent type of influenza circulating in the United States after causing flu outbreaks in other countries.
“In very short order, the new flu strain has become the dominant strain that’s out there circulating,” says William Schaffner, MD, an infectious disease specialist at Vanderbilt University Medical Center. “The reports across the country are that influenza is taking off like a rocket at the moment virtually everywhere. Our hospitalized cases here are climbing very rapidly and have for the last few weeks.”
Flu symptoms typically strike suddenly, a characteristic that distinguishes the flu from other infections, like COVID-19, Dr. Schaffner says. If you experience these symptoms, call your healthcare provider so you can initiate prompt antiviral treatment:
- Fever
- Chills
- Cough
- Sore throat
- Runny/stuffy nose
- Achiness
- Headaches
- Severe fatigue
- Vomiting
About the New Flu Strain
Flu season in the United States typically peaks between December and February, although it can vary from year to year. H3N2 is a subtype of influenza A virus and is a common cause of illness each flu season, along with influenza A subtype H1N1 and influenza B viruses.
According to the Centers for Disease Control and Prevention’s (CDC’s) FluView weekly influenza surveillance report for the week ending Dec. 20, 2025, H3N2 viruses were the most frequently reported influenza viruses of the flu season, comprising about 92% of the 1,627 influenza A viruses subtyped.
This flu season, H3N2 has developed a number of genetic changes, or mutations, resulting in the current H3N2 subclade K version of the virus. Of the 275 H3N2 viruses collected from Sept. 28, 2025, to Dec. 20, 2025, about 90% were subclade K, the CDC reported.
The CDC first identified subclade K in August 2025. The virus triggered outbreaks in Japan and England, and the World Health Organization (WHO) reported in December that subclade K accounted for as many as 90% of confirmed influenza cases in the European region. The WHO also noted that flu season began about four weeks earlier than in previous seasons, and at least 27 of the 38 countries in the region reported high or very high influenza activity.
Now, subclade K is making its way across Canada and the United States.
“Its striking characteristic is that it is highly contagious,” Dr. Schaffner says. “This virus has mutated. It’s now the most dominant strain, so it’s ‘outrunning’ all of the other strains.”
There’s no evidence that the new flu strain is considerably more severe or produces distinctive clinical syndromes, compared with earlier H3N2 or other influenza viruses, Dr. Schaffner notes (see “Identify Flu Symptoms”). However, H3N2 viruses tend to produce more severe illness, especially among adults, he adds.
“If you put all those things together, it’s clearly the one that is energizing this very substantial uptick in flu infections that we’re having,” Dr. Schaffner says. “We’re in the midst of what looks to be another very substantial flu season, which violates one of the flu’s rules, because we had a very big flu season last year. It’s very unusual to have two large flu seasons back to back, but we’re on our way to having a very substantial flu season two years in a row.”
Guard Against the New Flu Strain
The H3N2 subtype, along with other influenza A and B subtypes, is included in the seasonal influenza vaccine (flu shot). However, subclade K has mutated enough to make it different from the initial H3N2 virus that was used when this season’s vaccine was developed, raising uncertainty about the vaccine’s efficacy this year.
Fortunately, the WHO points to early data from the United Kingdom suggesting that the flu shot does provide some protection against the most serious outcomes of influenza—namely, hospitalizations and deaths.
So, if you haven’t already, it’s still worthwhile to get the current influenza vaccine, Dr. Schaffner advises. “Although subclade K is the dominant strain, there are H1N1 and influenza B viruses still out there. The vaccine will also provide protection against those other strains, which may become more prominent as the season evolves,” he says. “This is a long flu season, and you can get flu more than once with two different strains. It’s late, but not too late to get the vaccine. Now is the time to run, not walk, to your pharmacy or healthcare provider to get the vaccine.”
Generally, it takes 10 days to two weeks to gain maximum protection from the flu shot, although your immune system begins to ramp up its defenses against influenza the moment you’re vaccinated, Dr. Schaffner explains. Be aware that the vaccine does not necessarily prevent influenza altogether, but it can reduce your risk of severe flu outcomes.
“It’s not a perfect vaccine,” he adds. “It’s not as good at preventing the milder infections. Its job is to keep us out of the hospital. The vaccine seems to be doing its job in helping to keep people out of the hospital, the intensive care unit, and, as I like to say, the cemetery.”
Other Influenza Precautions
With the flu so prevalent, consider limiting or avoiding crowded indoor environments, where influenza readily spreads. If you’re in such an environment, avoid people who are coughing or sneezing, and wear a mask (covering your nose and mouth), Dr. Schaffner recommends. These precautions are especially important for people at higher risk of influenza-related complications, such as those with chronic lung or heart disease or autoimmune disorders, as well as immunocompromised individuals.
Importantly, if you develop flu symptoms, avoid contact with other people, and contact your health-care provider so you can receive antiviral therapy, particularly if you’re at high risk of complications. Four antiviral medications—baloxavir (Xoflusa), oseltamivir (Tamiflu), peramivir (Rapivab), and zanamivir (Relenza)—are recommended to treat flu this season.
“Generally, adults tend to put off calling the doctor, but that is not a recipe for success,” Dr. Schaffner advises. “Every day you put it off, once you start the antiviral, it reduces its effectiveness. So don’t hesitate. The antivirals work.
“First and foremost, we have to respect influenza. It’s serious, so get the vaccine, and if you haven’t, hurry up and get it,” he continues. “If you’re in a high-risk group, now’s the time to hunker down and avoid those indoor gatherings. If you do go, wear a mask, wash your hands frequently, and avoid people who are coughing or sneezing. And, should you develop symptoms, quickly call your health-care provider so they can initiate antiviral treatment.”
