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.Of 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.

(p^Hydroxychloroquinep^placebo))=0.024when rounded to 3 decimal places: you may verify this on your own if you wish.)