CDC: Flu strain doesn't match this year's vaccine

Federal health officials are warning that this year's flu vaccine will do little to stop about half of influenza cases in the United States.

The Centers for Disease Control and Prevention (CDC) alerted doctors Wednesday night that the predominant strain of flu this year — H3N2 — has mutated substantially enough not to match the vaccine.

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While the shot protects against three of four strains of influenza, including H3N2, roughly half of cases are now caused by mutations that don't match the treatment, according to NBC News.

The CDC is still encouraging people to get the flu shot in case they come down with a non-mutated strain. The agency suggested the vaccine could also help prevent "severe outcomes, such as hospitalization and death."

Influenza activity has increased in most parts of the country as of late November, the alert stated. Flu season typically peaks in the early part of the year.

CDC Director Tom Frieden said Thursday that it is too late to manufacture vaccines that could beat the mutations. The process takes about four months, he said.