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. 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.
