A new form of the flu, which is highly contagious and aggressive, is sweeping the nation this season.
A mutation of influenza A H3N2, called subclade K, has been detected as the culprit in rising global cases, including in the U.S.
The World Health Organization stated on its website that the K variant marks "a notable evolution in influenza A (H3N2) viruses," which some say calls into question the effectiveness of this season's influenza vaccine against the strain.
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The K variant causes more intense flu symptoms, including fever, chills, headache, fatigue, cough, sore throat and runny nose, according to experts.
Among 216 influenza A(H3N2) viruses collected since Sept. 28, 89.8% belonged to subclade K, a CDC report states.
The agency continues to release a weekly influenza surveillance report, tracking which states are seeing the most activity for outpatient respiratory illness.
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Below are the states and regions within the highest range of reported medical visits, not necessarily confirmed influenza cases, as of the week ending Dec. 13.
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The CDC has estimated that there have been at least 4.6 million illnesses, 49,000 hospitalizations and 1,900 flu deaths this season so far. The flu vaccine is recommended as the best line of defense against the virus.
In an interview with Fox News Digital, Dr. Neil Maniar, professor of public health practice at Boston's Northeastern University, shared details on the early severity of this emerging flu strain.
"It's becoming evident that this is a pretty severe variant of the flu," he said. "Certainly, in other parts of the world where this variant has been prevalent, it's caused some severe illness, and we're seeing an aggressive flu season already."
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Subclade K is the "perfect storm" for an aggressive flu season, Maniar suggested, as vaccination rates overall are down, and it's uncertain whether this year's flu vaccine directly addresses this specific mutation.
"The vaccine is very important to get, but because it's not perfectly aligned with this variant, I think that's also contributing to some degree to the severity of cases we're seeing," he said. "We're going in [to this flu season] with lower vaccination rates and a variant that in itself seems to be more aggressive."
Maniar stressed that it's not too late to get the flu vaccine, as peak flu season has not yet arrived.
"The vaccine still provides protection against serious illness resulting from the subclade K variant that seems to be going around," he said. "There are likely to be lots of indoor gatherings and other events that create risk of exposure, so protection is important."
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Even healthy individuals can become seriously ill from the flu, Maniar noted, "so a vaccine is beneficial for almost everyone."
"Individuals typically start to develop some degree of protection within a few days and gain the full benefit within about two weeks, so now is the time for anyone who hasn't gotten the vaccine yet."
