The experimental antiviral drug remdesivir has proven effective against COVID-19 in a small experiment involving monkeys, US government scientists reported Friday.

The study, which is preliminary and has not yet been peer reviewed, was designed to follow dosing and treatment procedures used for hospitalized COVID-19 patients in a large human trial.

It involved two groups of six rhesus macaques that were deliberately infected with SARS-CoV-2.

One group received the drug, which was developed by Gilead Sciences, and the other group did not. 

The group receiving the drug got their first intravenous dose 12 hours after infection then every day subsequently for six days.

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The scientists timed the initial treatment to occur shortly before the virus reached its highest levels in the animals' lungs.

The treated animals showed significant improvement 12 hours after their first dose, a trend that continued over the week-long study.

One of the six treated animals showed mild breathing difficulty, while all six of the untreated monkeys had rapid and difficult breathing. 

The amount of virus found in the lungs was significantly lower in the treated group compared to the untreated group.

The treated group also had less lung damage.

Remdesivir was among the first drugs mooted as a treatment for the novel coronavirus and its randomized clinical trials are at an advanced stage. 

The health news website Stat on Thursday reported the drug had shown great efficacy at a Chicago hospital where patients who are part of those trials are being treated. 

Remdesivir causes the virus to add mutations that can destroy it.