This is a blinded, multicenter, placebo-controlled randomized clinical trial. In this trial, the safety and effectiveness of hydroxychloroquine for the treatment of adults hospitalized with COVID-19 will be evaluated. In this study, a medication called hydroxychloroquine is being investigated to see if it improves recovery from COVID-19 in patients admitted to the hospital. Hydroxychloroquine is used to treat malaria and some joint (rheumatologic) diseases, but it is unknown if it helps patients recover from COVID-19. Study medication will be given for 5 days. This medication could be either hydroxychloroquine or placebo. There will be up to 510 patients in the study at about 50 hospitals in the United States.