Researchers used a randomized, double-blind, placebo-controlled study to investigate hydroxychloroquine sulfate as a potential postexposure prophylactic drug for COVID-19. After recruitment in the United States and Canada, 821 asymptomatic individuals who were exposed to COVID-19 were treated within 4 days of contact with infected individuals. Participants were considered highly exposed if they were less than 6 feet away from an individual with confirmed COVID-19 infection for more than 10 minutes without wearing a mask or eye shield. Moderately exposed individuals met all these requirements except they were wearing a mask during exposure. Participants were given either placebo (n=407) or hydroxychloroquine sulfate (n=414) tablets on a dosing regimen. They were monitored for the incidence of COVID-19 infection or related symptoms for 14 days. The results of the study showed that there was no significant difference between new illness and symptoms compatible with COVID-19 infection in the hydroxychloroquine sulfate and placebo groups (11.8% and 14.3%, respectively).