DENVER—The new and seemingly more contagious variant of the coronavirus that has set off the alarm in Britain has been reported for the first time in the United States, in a Colorado man who hadn’t been traveling, triggering a host of questions about how it got here and adding urgency to the nation’s vaccination drive. The patient, who is in his 20s, was recovering in isolation in Elbert County, a mostly rural expanse on the edge of the Denver metropolitan area, officials said Tuesday. His condition was not disclosed. The new, mutated version was first identified in Britain, where infections are soaring and the number of hospitalized COVID-19 patients has surpassed the first peak seen last spring. The variant has also been found in several other countries. “There is a lot we don’t know about this new COVID-19 variant, but scientists in the United Kingdom are warning the world that …