A randomized, double-blind, placebo-controlled
trial to test hydroxychloroquine as post-exposure prophylaxis found that it does not prevent illness.
Enrollment began on March 17th, 2020. To be eligible, people had be enrolled within 3 days after confirmed exposure. 821 asymptomatic participants, 719 who had reported a high-risk exposure to a confirmed Covid-19 contact, were enrolled.