Roughly half of the circulating influenza A viruses collected in the U.S. early this flu season are a different virus strain than included in this year’s vaccine, prompting CDC officials to remind healthcare providers about using medications like Tamiflu or Relenza aggressively to treat and prevent influenza.

“They’re different enough that we’re concerned that protection from vaccination may be lower than we usually see,” CDC Director Tom Frieden told reporters on Thursday.

The CDC is still recommending that people get vaccinated against the flu because it provides partial protection and the B strains are well matched. But Frieden said that if clinicians suspect influenza in high-risk patients, they should start Tamifu without waiting for confirmatory test results.

CDC health advisory