Just hours after the WHO announced that clinical trials of hydroxychloroquine will resume, the first major study into the drug's effectiveness reported that it was no better than a placebo in preventing symptoms of Covid-19 developing.
Hydroxychloroquine has been controversially endorsed by public figures including US President Donald Trump despite a lack of evidence that it is effective.
University of Minnesota researchers tested 821 people who had recently been exposed to the virus or lived in a high-risk household.
It found 11.8% of subjects given hydroxychloroquine developed symptoms compatible with Covid-19, compared with 14.3% who got a placebo.