The true number of coronavirus cases in the U.S. could be anywhere from six to 24 times higher than the confirmed number of cases, depending on location, according to a large federal study that relied on data from 10 U.S. cities and states. The vast majority of Americans, however, are still vulnerable to Covid-19. The study, published Tuesday in JAMA Internal Medicine, relied on serological tests — blood screens that search for antibodies to the virus and that determine whether someone was previously infected. … In seven of the 10 sites, the estimated number of cases was 10 times the number of reported cases.