Britain on Thursday became the first country to approve an anti-Covid pill, as it greenlit the use of Merck's antiviral drug to treat patients suffering from mild to moderate coronavirus, regulators said. 

"Today is a historic day for our country, as the UK is now the first country in the world to approve an antiviral that can be taken at home for Covid-19," said health minister Sajid Javid.

"This will be a game-changer for the most vulnerable and the immunosuppressed, who will soon be able to receive the ground-breaking treatment," he added.

The antiviral, called molnupiravir, works by decreasing the ability of a virus to replicate, thereby slowing down the disease.

The Medicines and Healthcare products Regulatory Agency (MHRA) said its trials had concluded it was "safe and effective at reducing the risk of hospitalisation and death in people with mild to moderate Covid-19 who are at increased risk of developing severe disease".

Based on the clinical trial data, the drug is most effective when taken during the early stages of infection and the MHRA recommends that it be used within five days of the onset of symptoms.

It has been authorised for use in people who have at least one risk factor for developing severe illness, including obesity, old age, diabetes and heart disease.

Britain, which has been one of the countries hardest hit by the pandemic, announced on October 20 that it had ordered 480,000 doses of molnupiravir from US pharma giant Merck.

Drug regulators in the United States and the European Union have already begun an evaluation of the drug.

Not a vaccine replacement 

Merck has already signed agreements with other governments, including the US, which has planned to buy 1.7 million doses if molnupiravir if approved by regulators.

MHRA chief executive June Raine called the pill "another therapeutic to add to our armoury". 

"It is also the world's first approved antiviral for this disease that can be taken by mouth rather than administered intravenously," she added. 

"This is important, because it means it can be administered outside of a hospital setting."

Clinical trials found the drug to be effective in reducing the risk of hospital admission or death for at-risk non-hospitalised adults by 50 percent, according to Munir Pirmohamed, chair of the Commission on Human Medicines.

But experts have warned that the treatment is not a miracle cure and Pirmohamed said it was not intended to be used as a substitute for vaccination against the virus.

The government said that it and the state-run National Health Service will announce the rollout of the treatment "in due course".

A simple pill to treat the coronavirus has been sought since the start of the pandemic and Merck's announcement of its trial results was hailed as a major step towards that goal.

Until now, Covid therapeutics such as monoclonal antibodies and Gilead's remdesivir -- authorised for use in the EU under the name Veklury -- have been administered intravenously.

Molnupiravir was initially developed as an inhibitor of influenza and respiratory syncytial virus -- two other important acute respiratory infections -- by a team at Emory University in Atlanta, Georgia.

Merck is also conducting a separate clinical trial for a second use of the drug, as a measure to prevent people who have been in close contact with those carrying the virus from developing it.