“We’re clearly turning the corner,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Across the country, the outlook for the pandemic has indeed improved, putting the United States in its best position against the virus yet. The nation is recording about 49,000 new cases a day, the lowest number since early October, and hospitalizations have plateaued at around 40,000, a similar level as the early fall. Nationwide, deaths are hovering around 700 a day, down from a peak of more than 3,000 in January... In the United States, even as a sense of hope spreads, there remain strong reasons for caution. The pace of vaccinations is slowing, and experts now believe that herd immunity in the United States may not be attainable. More transmissible variants of the virus are also spreading, threatening to undermine the progress from vaccinations. That could leave the coronavirus infecting tens of thousands of Americans and killing hundreds more each day for some time. A modeling study released by the Centers for Disease Control and Prevention on Wednesday, citing relaxed restrictions and a new, contagious variant, suggested that cases could tick upward again in the coming weeks, before a sharp drop-off by July. Dr. Rochelle Walensky, the director of the C.D.C., said, “We are not out of the woods yet, but we could be very close.”