By Deidre McPhillips

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 US Centers for Disease Control and Prevention.

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

“There’s active global surveillance going on looking for these mutations,” said Dr. Andy Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health. “We want to know when they come up, because that could really have major implications for how we treat influenza.”

For now, though, experts say the threat is low.

Cases of the “dual mutant” influenza variant were identified in 15 countries across five continents, including two cases in the US, according to a report published Wednesday in the CDC’s Emerging Infectious Diseases journal. But they’re very infrequent, representing only about 1% of the samples collected between May 2023 and February 2024.

“They appear in a lot of places, but they’re never really the dominant virus in any place they appear,” Pekosz said.

“It’s not like the mutation occurred someplace, and suddenly that particular virus began spreading and out-competing everything in one big wave,” he said. Instead, the same mutations seem to be developing in multiple places independently.

The doubly mutated variant has shown a reduction in Tamiflu effectiveness of up to 16-fold, according to the CDC report – but that’s based on laboratory studies.

Other influenza variants have shown much greater resistance in the lab, and there’s no gauge for what this could mean in real-life clinical practice, said Dr. William Schaffner, an infectious disease expert at Vanderbilt University.

“We don’t know really what the clinical significance of this slight resistance is,” he said. “It may well be that the amount of Tamiflu that we give patients is sufficient to overcome this.”

Laboratory testing suggests that other antiviral treatments – including a newer antiviral called baloxavir marboxil, or Xofluza – are still effective against the dual mutants, according to the CDC.

And flu vaccines should offer protection against infections caused by viruses with the mutations, too.

“There are no immediate implications with regard to clinical care decisions,” the CDC said in an emailed statement. “CDC is continuously monitoring the antiviral susceptibility of circulating seasonal influenza viruses in the U.S. and worldwide, along with global partners. This global virologic surveillance will inform the potential public health threat of these influenza viruses.”