This year’s flu vaccine is not as effective against the current strain of the influenza virus because the virus has mutated, the U.S. Centers for Disease Control and Prevention said Wednesday in an advisory to doctors. The advisory said 52% of the 85 influenza virus samples collected and analyzed from October 1 through November 22 were different than the virus strains included in this year’s vaccine, indicating a mutation, or drift, of the strain.

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