The medicine has been touted by US President Donald Trump, who has said he used it as a prophylaxis against the novel coronavirus.

But an experiment involving 821 people across the United States and Canada showed it did not work significantly better than a placebo for this purpose.

The study was led by a team at the University of Minnesota, and their paper was published in the New England Journal of Medicine.

Researchers enrolled adults who had come into contact with someone who had a confirmed case of COVID-19 for more than 10 minutes at a distance of about two metres or less.

The majority of them - 719 - were deemed to have had "high-risk" exposure because they wore neither a face mask nor an eye shield at the time, while the rest were "moderate-risk" because they covered their face but did not have goggles.

"This randomised trial did not demonstrate a significant benefit of hydroxychloroquine as postexposure prophylaxis for COVID-19," wrote the authors.

The results of the study were eagerly awaited because it was a randomised controlled trial (RCT), a carefully designed experiment that is considered the gold standard for investigating clinical outcomes. 

Several previous studies on the drug that have made headlines were "observational," meaning they looked back at what had already happened. As such, more variables are left to chance and it is generally harder to draw firm conclusions. 

Nevertheless, Martin Landray, a professor of medicine and epidemiology at the University of Oxford, said more research was needed to know for sure whether hydroxychloroquine might have a moderately positive effect.

"The study is too small to be definitive," said Landray, who was not involved in the trial.

The results "makes it very unlikely that there is a large effect (eg a halving in the risk of infection) but cannot rule out a more modest difference (eg a reduction of one-quarter or one-third) which would still be very valuable," he added.

In the meantime, the World Health Organisation announced that clinical trials of the drug hydroxychloroquine will resume as it searches for potential coronavirus treatments.

On May 25, the WHO announced it had temporarily suspended the trials to conduct a safety review, which has now concluded there is "no reason" to change the way the trials are conducted.

The UN health agency's decision came after a study published in The Lancet medical journal suggesting the drug could increase the risk of death among COVID-19 patients.

The executive group of the so-called Solidarity Trial - in which hundreds of hospitals across the world have enrolled patients to test several possible treatments for the novel coronavirus - took the decision as a precaution.

Hydroxychloroquine is normally used to treat arthritis but public figures including US President Donald Trump have backed the drug for COVID-19 prevention and treatment, prompting governments to bulk-buy.

"Last week, the executive group of the Solidarity Trial decided to implement a temporary pause of the hydroxychloroquine arm of the trial, because of concerns raised about the safety of the drug," WHO chief Tedros Adhanom Ghebreyesus told a virtual news briefing.

"This decision was taken as a precaution while the safety data were reviewed.

"The data safety and monitoring committee of the Solidarity Trial has been reviewing the data.

"On the basis of the available mortality data, the members of the committee recommended that there are no reasons to modify the trial protocol.

"The executive group received this recommendation and endorsed continuation of all arms of the Solidarity Trial, including hydroxychloroquine.

"The executive group will communicate with the principal investigators in the trial about resuming the hydroxychloroquine arm of the trial.

"The data safety and monitoring committee will continue to closely monitor the safety of all therapeutics being tested in the Solidarity Trial."

More than 3,500 patients have been recruited across 35 countries to take part in the trials.

eNCA/AFP