https://www.wired.com/story/major-hydroxychloroquine-trial-shows-no-prevention-benefits/

The drug hydroxychloroquine, touted by President Donald Trump and his allies as a treatment and preventative for the pandemic disease Covid-19, does not keep people from getting sick. A large, randomized study, scheduled to appear today in The New England Journal of Medicine shows that the drug is no better than a placebo at preventing infection in people exposed to the virus under real-world conditions.

This was a careful, well-planned trial—one of the most rigorous tests of hydroxychloroquine yet completed. Researchers based at the University of Minnesota looked at 821 otherwise healthy individuals in the US and Canada, all of whom had confirmed exposures to people who tested positive for infection with the virus SARS-CoV-2, which causes Covid-19. Half of the subjects got relatively high doses of hydroxychloroquine within four days of exposure, couriered from a pharmacy. The other half, the control group, received placebo pills that looked similar but had no active drug. Since this study was “double-blind,” only the pharmacists knew who got which pills; the researchers only saw the results. The researchers kept in touch over two weeks, at which point they asked: Who had Covid-19?