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According to the CDC, a “double mutant” strain of influenza has been identified in 15 countries, including two in the United States.
CNN
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A rare variant of the flu that appears to be somewhat resistant to the most commonly used antiviral treatments has been detected in at least two people in the United States, according to the Centers for Disease Control and Prevention.
Influenza viruses are constantly changing, but this variant has two worrying mutations that could reduce the effectiveness of treatment with oseltamivir phosphate, known by the brand name Tamiflu.
“There’s active surveillance going on around the world looking for these mutations,” said Dr. Andy Pecos, a virologist at the Johns Hopkins Bloomberg School of Public Health. “We want to know when they emerge, because that could have a big impact on how we treat influenza.”
But for now, experts say the threat is low.
Cases of the “double mutation” influenza variant have been confirmed in 15 countries across five continents, including two in the United States. report The study was published Wednesday in the CDC’s Emerging Infectious Diseases journal. But its frequency is very low, accounting for just about 1% of samples collected from May 2023 to February 2024.
“They pop up in a lot of places, but they never really become the dominant virus in the places where they pop up,” Pecos said.
“It’s not like a mutation occurred somewhere and all of a sudden that particular virus started spreading like wildfire and overwhelming everything,” he said. Rather, the same mutation appears to have arisen independently in multiple places.
The CDC report suggests the double mutant strain may make Tamiflu up to 16 times less effective, but that’s based on laboratory studies.
Dr. William Schaffner, an infectious disease expert at Vanderbilt University, said other flu variants have shown much greater resistance in the lab, and there’s no benchmark to gauge what that means in real-world clinical practice.
“We’re not really sure what the clinical significance of this slight resistance is,” he said, “but it’s quite possible that the amount of Tamiflu we give to patients is enough to overcome this resistance.”
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According to the CDC, laboratory tests suggest that other antiviral treatments, including a newer antiviral drug called baloxavir marboxil (also known as Xofluza), remain effective against the double mutant.
In addition, influenza vaccines should be effective in preventing infections caused by mutated viruses.
“There is no immediate impact on clinical treatment decisions,” the CDC said in an emailed statement. “CDC, along with partners around the world, continues to monitor the antiviral susceptibility of circulating seasonal influenza viruses in the United States and around the world. This global virological surveillance will inform us of the potential public health threat of these influenza viruses.”
CNN’s Maya Davis contributed to this report.
