Hydroxychloroquine, the drug backed by President Donald Trump to combat COVID-19, is no better than a placebo in preventing infection of the coronavirus, according to results published Wednesday in the New England Journal of Medicine.

The study, the first randomized, placebo-controlled trial, which is considered the "gold standard" in science, looked at 821 people in the United States and Canada who had been exposed to the coronavirus.

The researchers, led by Dr. David Boulware, an infectious disease researcher at the University of Minnesota, said the drug was given to 414 people for five days while 407 got a placebo. Nearly 80% of the patients reported a high-risk exposure to a confirmed COVID-19 contact. 

About 12% of the people who were given the malaria drug developed COVID-19, compared with 14% who did not receive the drug, according to the study's findings, which included health-care workers and people close to those with COVID-19. They said side effects were more common with those who took hydroxychloroquine, but there were no "serious" adverse reactions. Results were no different for those using zinc and vitamin C, the researchers said. 

No deaths were reported. About three-quarters of the patients had no underlying health conditions, according to the study, while the rest had reported hypertension, asthma, or diabetes. 

The coronavirus, which emerged in Wuhan, China, about five months ago, has sickened more than 6 million people worldwide and killed at least 382,299, according to data compiled by Johns Hopkins University.

In addition to treating malaria, hydroxychloroquine is often used by doctors to treat rheumatoid arthritis and lupus. It is known to have serious side effects, including muscle weakness and heart arrhythmia. Numerous clinical trials are looking to see if it's effective in fighting Covid-19, but it is not a proven treatment.

Even though the drug is not a proven treatment for the coronavirus, some people across the world have been taking it after a handful of small studies published earlier in the year suggested it could be beneficial and Trump promoted the drug as a potential treatment for the virus.

A recent study published in JAMA found that U.S. prescriptions for the malaria drug surged nearly 2,000% in March when Trump first promoted the drug as a potential treatment for the coronavirus.

Trump disclosed last month he was taking hydroxychloroquine daily to prevent infection from the coronavirus. White House physician Dr. Sean Conley released a memo that said that after discussing evidence for and against hydroxychloroquine with Trump, they concluded "the potential benefit from treatment outweighed the relative risks."

Also Wednesday, the World Health Organization said it was resuming its trial of hydroxychloroquine after temporarily halting research over safety concerns.

On May 25, the WHO announced it had temporarily suspended its trial of the drug. The announcement came days after a study published in the medical journal The Lancet found that hospitalized COVID-19 patients treated with hydroxychloroquine had a higher risk of death than those who didn't take it. However, that study is now being reexamined over concerns about fabricated data.