This randomized trial showed that hydroxychloroquine did not prevent presumed/confirmed COVID-19 when used as post-exposure prophylaxis within 4 days of high- or moderate-risk COVID-19 exposure.

• This randomized, double-blind, placebo-controlled trial occurred across the United States and parts of Canada
• Enrolled adults who had household or occupational exposure to someone with confirmed Covid-19 at a distance of less than 6 ft for more than 10 minutes while wearing neither a face mask nor an eye shield (high-risk exposure) or while wearing a face mask but no eye shield (moderate-risk exposure).
• Within 4 days after exposure, participants were randomly assigned to receive either placebo or hydroxychloroquine (800 mg once, followed by 600 mg in 6 to 8 hours, then 600 mg daily for 4 additional days).
• 821 asymptomatic participants were enrolled, most of whom (87.6) had a high-risk exposure
• The incidence of presumed/confirmed COVID-19 within the 14 day follow-up period did not differ significantly between groups:
  • Hydroxychloroquine (49 of 414 [11.8%]) and Placebo (58 of 407 [14.3%])
  • Absolute difference was −2.4 percentage points (95% confidence interval, −7.0 to 2.2; P=0.35).
• Side effects were more common with hydroxychloroquine than with placebo (40.1% vs. 16.8%), but no serious adverse reactions were reported