CDC warns of ‘dual mutant’ flu strain that could evade antiviral drugs: ‘Need to closely monitor’

A new "dual mutant" strain of H1N1 influenza could pose a threat in the U.S., according to the Centers for Disease Control and Prevention (CDC).

Details about the two strains, I223V and S247N, were published in the journal Emerging Infectious Diseases, which is published by the CDC.  

At least two cases of the flu mutations have been confirmed in humans in the U.S., the agency announced on Wednesday.

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The researchers identified a total of 101 samples of the "dual mutant" virus.

The concern is that these strains of flu have shown to be resistant to the antiviral medications that are typically used to treat the virus — notably Tamiflu (oseltamivir), the common flu medication from Switzerland-based Roche.

A new "dual mutant" strain of H1N1 influenza could pose a threat in the U.S., according to the Centers for Disease Control and Prevention. (iStock)

"In laboratory tests, these viruses were found to be about 13 to 16 times less sensitive to oseltamivir than those without this combination of mutations," a CDC spokesperson told Fox News Digital.

"For perspective, another mutation called H275Y makes the virus 500 to 1,000 times less sensitive to oseltamivir in the laboratory test."

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Given that these mutated viruses were still sensitive to other anti-influenza medications, including baloxavir marboxil, the CDC spokesperson said that "there are no immediate implications to change decisions for clinical care." 

"Our study highlights the need to closely monitor the evolution of dual mutants, because additional changes may further affect susceptibility to antiviral drugs or provide a competitive advantage over circulating wild-type viruses," the researchers wrote in the study findings.

The strains have been detected in 15 countries across five continents, but are mostly prevalent in Europe, the study found.

"The dual mutants that we tested retained susceptibility to other approved influenza antiviral drugs, including baloxavir," the researchers wrote. (iStock)

The researchers determined that the mutations have been circulating globally since May 2023.

I223V and S247N were first tested by Hong Kong scientists, who published their findings in The Lancet Microbe in March 2024. 

Those researchers also found that the mutant strains lessened the effectiveness of Tamiflu.

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Dr. Marc Siegel, clinical professor of medicine at NYU Langone Medical Center and a Fox News medical contributor, said it is "highly unusual" to see an influenza A strain (H1N1, which is a distant cousin of the Spanish flu, he noted) spreading throughout southern U.S. states at this time of year. 

People who contract the flu can take antiviral medications to ease symptoms, the CDC stated, which is most important for high-risk patients. (iStock)

"Flu season is usually over by March," Siegel told Fox News Digital. "Flu doesn't spread as easily in hot, humid weather."

He added, "This mutation worries me because the use of Tamiflu — especially in high-risk groups and the elderly — is really helpful at decreasing severity, and it looks like the reaction to it is less."

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The doctor said he would still recommend using the drug, however, because it has some impact — "or consider other anti-flu drugs."

Siegel also advised a vaccine booster for the elderly or those in high-risk groups.

So far this season, the CDC estimates that there have been at least 35 million illnesses, 390,000 hospitalizations and 24,000 deaths from the flu, as published on the agency’s website.

The CDC recommends that everyone 6 months and older receives an annual vaccine during heightened flu spread. (iStock)

The CDC recommends that everyone 6 months and older receives an annual vaccine during heightened flu spread.

People who contract the flu can take antiviral medications to ease symptoms, the CDC stated, which is most important for high-risk patients.

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"It is unknown how widely these mutated viruses will circulate in the upcoming season," the CDC told Fox News Digital. 

"It is important to continue monitoring the spread … and the evolution of these viruses, as they may acquire other mutations that could further impact the usefulness of oseltamivir or other antiviral medications."

Fox News Digital reached out to Roche for additional comment.