In a study of more than 800 asymptomatic patients in the US and Canada, researchers at the University of Minnesota found that hydroxychloroquine did not protect them better from Covid-19 than placebo after 4 days of either high and moderate risk of exposure.

Health care workers accounted for 66.4% of the participants in this study, with 62.8% of them being physicians and 23.5% nurses.

The study, which was published in the New England Journal of Medicine, found that 107 of the 821 participants developed Covid-19 in the 14 of follow-up; this was split evenly between the hydroxychloroquine and placebo groups.

“This randomized trial did not demonstrate a significant benefit of hydroxychloroquine as postexposure prophylaxis for Covid-19,” the researchers concluded. “Whether preexposure prophylaxis would be effective in high-risk populations is a separate question, with trials ongoing.”

Last week, the UK’s healthcare regulator paused the University of Oxford’s international CopCov study of hydroxychloroquine and chloroquine to prevent Covid-19 in healthcare workers. This was linked to concerns about the efficacy and safety of the drug based on results of studies in the Lancet.