Much of the influenza virus circulating in the United States has mutated and this year's vaccine doesn't provide good protection against it, federal health officials are warning.
Flu season's barely starting, but most cases are being caused by a strain called H3N2 this year, the Centers for Disease Control and Prevention said in a health warning issued to doctors Wednesday night.
The flu vaccine protects against three or four strains of flu—there's always a mix of flu viruses going around—and H3N2 is one of them. But the strain of H3N2 infecting most people has mutated and only about half of cases match the vaccine, CDC said.
Flu viruses do this all the time. The mutations are called "drift" and vaccines, formulated months in advance, often don't protect well against drifted viruses although they may provide a little of what's called cross protection.
"We are recommending strongly still that people who haven't been vaccinated get vaccinated. Every year vaccine is the best way to protect yourself against the flu," the CDC's flu expert Dr. Joe Bresee told NBC News.
"Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death," CDC said in a health advisory to doctors. "And the vaccine still protects against half the circulating H3N2,as well as H1N1 flu and the B strains.