The number of deaths per day from the coronavirus in the U.S. has fallen in recent weeks to the lowest level since late March, even as states increasingly reopen for business. But scientists are deeply afraid the trend may be about to reverse itself. … Deaths from COVID-19 across the country are down to about 680 a day, compared with around 960 two weeks ago, according to an AP analysis of data compiled by Johns Hopkins University. … A multitude of reasons are believed to be at play, including the advent of effective treatments and improved efforts at hospitals and nursing homes to prevent infections and save lives. But already there are warning signs.