October 6, 2015
Every winter, scientists from WHO observe which strains of influenza circulate around the world. Based on WHO’s recommendation, the CDC chooses which viruses to target in the influenza vaccine for the United States.
During the 2014-2015 flu season, the dominant H3N2 influenza strain mutated after the flu vaccine was released. The strain used for last season’s vaccine did not match the circulating strain, the CDC reported, and as a result the vaccine reduced flu visits to doctors by only 19% (95% CI, 7%-29%). Typically, the vaccine reduces flu visits by 50% to 60%, according to the CDC.