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The strain of the flu virus now circulating throughout the United States has mutated and may evade this year’s vaccine, according to a health warning from the Centers for Disease Control and Prevention. A warning issued to doctors Wednesday night warned that one of the three or four strains of flu, H3N2, has mutated and about half of cases no longer match the vaccine. However, the vaccine may still provide some “cross protection” against the mutated, or “drifted” viruses. “Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death,” the CDC said in the advisory to doctors. “And the vaccine still protects against half the circulating H3N2, as well as H1N1 flu and the B strains.” So the advice still stands: Everyone six months and older should get a flu vaccine every year.