While the numbers are going in the right direction, they are still at once unimaginably high levels. … Experts fear the decline might just be temporary. A more transmissible — and, as evidence increasingly suggests, a seemingly deadlier — form of the coronavirus called B.1.1.7 is starting to build up in the U.S., even as overall case numbers come down. It’s not clear whether the variant could cause cases — and thus hospitalizations and deaths — to increase once again, but the specter of it is adding pressure to vaccinate as many people as quickly as possible and to drive down overall cases as low as possible.