In fact, B.1.1.7 seems to have the edge over nearly every variant identified so far. At a congressional hearing on Tuesday, Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, said B.1.1.7 made up 72 percent of cases in the country. “We’re really seeing B.1.1.7 pushing out other variants decisively,” said Emma Hodcroft, an epidemiologist at the University of Bern. The variants identified in California and New York turned out to be only moderately more contagious than older versions of the virus, and much of their initial success may have been luck. The overall boom in cases last fall amplified what might otherwise have gone undetected.