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The Centers for Disease Control and Prevention warned that a rare strain of the flu virus that is resistant to antiviral treatments has been identified in the United States.
The new strain has two distinct mutations that make it resistant to oseltamivir phosphate, which is sold under the brand Tamiflu.
The "dual mutant" virus was identified in two patients in the United States. It was also found in 14 other countries across five continents.
"There's active global surveillance going on looking for these mutations," Dr. Andy Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, told CNN. "We want to know when they come up, because that could really have major implications for how we treat influenza."
The drug-resistant strain of the flu virus is still rare, and health officials haven't raised the alarm yet.
"They appear in a lot of places, but they're never really the dominant virus in any place they appear," Pekosz added. "It's not like the mutation occurred someplace, and suddenly that particular virus began spreading and out-competing everything in one big wave."