A drug named molnupiravir developed in the US, licensed for production to five Indian companies and described as the first oral direct-action antiviral agent against the novel coronavirus is “highly effective” in reducing viral loads in Covid-19 patients, researchers said on Friday.

The US-based researchers, announcing their clinical trial results, said the findings demonstrate the safety and tolerability and antiviral efficacy of molnupiravir to reduce the replication and accelerate the clearance of SARS-CoV-2, the virus that causes Covid-19.

“The trial provides strong biological evidence to support development of molnupiravir as an oral agent to reduce viral replication and interrupt progression of Covid-19 in the early stages of the disease,” the researchers said in a paper not peer-reviewed yet but posted on medRxiv, an online preprint server.

Researchers say antivirals need to be administered during the early phase of the infection. In the US clinical trial, participants were eligible to join if they tested positive for SARS-CoV-2 infection within 96 hours and had symptoms within seven days of start of treatment.

Among 202 participants involved in the clinical trial, virus isolation was significantly lower among participants who received 800mg molnupiravir compared with participants who received a placebo used in drug trials to measure effect of experimental therapies. On day 5, the virus was not isolated from any participant who had received 400mg or 800mg molunpiravir — which essentially means they tested negative — compared with 11 per cent of those who had received the placebo.

Molnupiravir is the first oral direct-acting antiviral shown to be “highly effective” at reducing nasopharyngeal SARS-CoV-2 infectious virus and viral genetic material, the researchers at the University of North Carolina and Chapel Hill and Ridgeback Biotherapeutics said in their paper.

A public health specialist in India not associated with the study said the results looked “significant” but a more rigorous assessment of molnupiravir is expected to emerge from an ongoing larger trial that will evaluate the drug’s effect on symptom duration, severity and hospitalisations.

Researchers at Emory University in the US had first identified molnupiravir as a potential antiviral agent about two decades ago and studied its actions against influenza but the focus of testing turned to SARS-CoV-2 last year.

The drug is currently under development jointly by Ridgeback Biotherapeutics and Merck.

In April this year, Merck announced it had signed pacts with five Indian drug makers — Cipla, Dr Reddy’s Laboratories, Emcure Pharmaceuticals, Hetero Labs and Sun Pharmaceuticals — to provide licences to them to make molnupiravir for India and over 100 low- and middle-income countries.

Doctors say the drug taken as a pill is much easier to administer than other approved treatments such as the antiviral remdesivir or lab-made proteins called monoclonal antibodies which are given through intravenous injections.