Rare drug-resistant flu variant identified in US, CDC says

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A rare flu variant that has shown some resistance to the most commonly used antiviral treatment has been detected in at least two people in the United States, according to the U.S. Centers for Disease Control and Prevention.

Flu viruses are constantly evolving, but this variant has two concerning mutations in some places that could reduce the effectiveness of treatment with oseltamivir phosphate, known by the brand name Tamiflu.

“There is active global surveillance underway to look for these mutations,” said Dr. Andy Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health. “We want to know when they occur, because this could have major implications for how we treat influenza. »

But for now, experts believe the threat is low.

Cases of the “double mutant” flu variant have been identified in 15 countries on five continents, including two cases in the United States, according to a report published Wednesday in the CDC's journal Emerging Infectious Diseases. But they are very rare, representing only around 1% of samples collected between May 2023 and February 2024.

“They appear in many places, but they are never really the dominant virus where they appear,” Pekosz said.

“It’s not like the mutation happened somewhere and all of a sudden this particular virus started spreading and overtaking everything in one big wave,” he said. Instead, the same mutations appear to develop independently in multiple locations.

The doubly mutated variant showed a reduction in the effectiveness of Tamiflu by up to 16 times, according to the CDC report – but that is based on laboratory studies.

Other flu variants have shown much greater resistance in the laboratory, and there is no measure of what that might mean in actual clinical practice, said Dr. William Schaffner, an infectious disease expert at the Vanderbilt University.

“We don't really know what the clinical significance of this mild resistance is,” he said. “It may well be that the amount of Tamiflu we give to patients is enough to overcome this problem. »

Lab tests suggest that other antiviral treatments — including a newer antiviral called baloxavir marboxil, or Xofluza — are still effective against double mutants, according to the CDC.

And flu vaccines should also provide protection against infections caused by viruses with mutations.

“There are no immediate implications regarding clinical care decisions,” the CDC said in an emailed statement. “CDC continuously monitors the antiviral susceptibility of seasonal influenza viruses circulating in the United States and internationally, in collaboration with global partners. This global virological surveillance will shed light on the potential threat to public health from these influenza viruses.

CNN's Maya Davis contributed to this report.