A new strain of influenza is emerging as a serious health threat
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A new form of influenza, which is highly contagious and aggressive, is sweeping the country this season.
A mutation in the influenza A H3N2 virus, called subtype K, has been discovered as the reason behind the rise in global cases, including in the United States.
The K variant represents a “marked evolution in influenza A (H3N2) viruses,” which some say calls into question the effectiveness of this season’s flu vaccine against the strain, the World Health Organization said on its website.
A new, “aggressive” type of influenza is sweeping the world, while doctors warn of severe symptoms
Type K causes more severe flu symptoms, including fever, chills, headache, fatigue, cough, sore throat and runny nose, according to experts.
Multiple states in the Northeast reported higher rates of respiratory illnesses this month. (Istock)
Of the 216 influenza A(H3N2) viruses collected since Sept. 28, 89.8% belong to subclass K, according to the CDC report.
The agency continues to issue a weekly influenza surveillance report, tracking states with the greatest outpatient respiratory illness activity.
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Below are the states and territories that fall within the highest range of reported medical visits, not necessarily confirmed influenza cases, as of the week ending December 13.
CDC’s Weekly Influenza Surveillance Report is a map of outpatient respiratory illness activity, based on data reported to ILINet for the week ending December 13, 2025. (CDC Fluvio)
Very high (level 1)
Very high (level 2)
Very high (level 3)
- New Jersey
- rhode island
- Louisiana
- Colorado
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High (Level 1)
- Massachusetts
- Connecticut
- Michigan
- Idaho
- South Carolina
High (Level 2)
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High (Level 3)
- Washington, DC
- Maryland
- North Carolina
- Georgia
The CDC has estimated that there have been at least 4.6 million illnesses, 49,000 hospitalizations and 1,900 deaths due to influenza so far this season. The influenza vaccine is recommended as the best line of defense against the virus.
The influenza vaccine is recommended as the best line of defense against the virus. (Istock)
In an interview with Fox News Digital, Dr. Neil Maniar, professor of public health practice at Northeastern University in Boston, shared details about the early severity of the emerging flu strain.
“It has become clear that this is a very dangerous type of influenza,” he said. “Certainly, in other parts of the world where this variant has spread, it has caused some serious illness, and we are already seeing an aggressive flu season.”
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Manyar suggested that Subclade K is the “perfect storm” for an aggressive flu season, as vaccination rates have declined overall, and it is uncertain whether this year’s flu vaccine directly addresses this specific mutation.
“It’s very important to get the vaccine, but because it’s not exactly consistent with this variant, I think that also contributes somewhat to the severity of the cases that we’re seeing,” he said. “We’re going in [to this flu season] With lower vaccination rates and a variant that itself appears more aggressive.”
Subclade K “has caused some serious illness, and we’re already seeing a severe flu season,” one doctor said. (Istock)
Manyar stressed that it is not too late to get the influenza vaccine, as the peak flu season has not yet arrived.
“The vaccine still provides protection against serious disease caused by subtype K, which appears to be circulating,” he said. “There are likely to be a lot of indoor gatherings and other events that create exposure risk, so protection is important.”
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Even healthy individuals can become seriously ill with influenza, “so the vaccine is beneficial for almost everyone,” Maniar noted.
“Individuals typically start to develop some degree of protection within a few days and get the full benefit within about two weeks, so this is a good time for anyone who hasn’t gotten the vaccine yet.”