A July 2020 article in the New England Journal of Medicine reports the results of a randomized trial designed to assess the efficacy of Hydroxychloroquine in preventing COVID-19 infection in persons exposed to others with COVID-19.

There were 821 subjects enrolled in this study. If the 414 randomized to the treatment (Hydroxychloroquine) arm, 49 (11.8%) developed COVID (or probable COVID) within 14 days of randomization. Of the 407 randomized to the placebo arm, 58 (14.3%) developed COVID (or probable COVID) within 14 days of randomization.

Estimate and report a 95% confidence interval for the relative risk from item 27.

RR= 0.118/0.143= 0.83