CDC: Flu shot only 12 percent effective for adults

This season's flu vaccine was only 12 percent effective so far for adults ages 18 to 49 and 14 percent effective for people 50 and older, the Centers for Disease Control and Prevention (CDC) estimated Thursday.

In its weekly Morbidity and Mortality Report, the CDC acknowledged that this season's shot offered less protection because about 70 percent of circulating flu viruses are different or have "drifted" from the one used to create the vaccine.

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The evolution of viruses cannot be helped and the composition for this year's vaccine had already been chosen before there was evidence of "drift," the CDC said.

The news underscores health officials' challenge in encouraging people to get vaccinated for the flu. Doctors report that frustrated patients have pointed to this year's lower effectiveness rate as a reason not to receive the shot.

The CDC recommended Thursday that clinicians prepare to use additional flu treatment measures than would normally be necessary, especially for vulnerable patients.

"All hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient’s vaccination status," said Joe Bresee, branch chief in the CDC's flu division, in a statement.

The CDC also noted that the effectiveness of the flu vaccine can vary based on a person's age and health. Children ages 17 and younger saw the greatest benefit from the shot, which was 26 percent effective among that group, according to early estimates.

The United States is seeing a "moderately severe" flu season, the agency said.