UK variant (B.1.1.7) The U.K. variant was one of the first identified "variants of concern." It's estimated to have first emerged in September, and it quickly gained a foothold, spreading like wildfire throughout the U.K. and then other parts of the world. At least 293 cases have been reported in the U.S., according to the Centers for Disease Control and Prevention, which said the actual tally probably exceeds that. There's good evidence this variant is more transmissible, spreading more easily and more quickly. This increased transmissibility is believed to be due to mutations in the spike protein -- the part of the virus that allows it to "stick" to cells. This spike protein is what the vaccines target.