The UK medicines regulator has become the first in the world to approve an oral antiviral pill for Covid in a move that paves the way for tens of thousands of vulnerable patients to receive the treatment from this winter.

Nearly half a million doses of molnupiravir, a pill that can be taken twice daily at home, are due for delivery from mid-November and will be given as a priority to elderly Covid patients and those with particular vulnerabilities, such as weakened immune systems.

Prof Peter Horby, the co-leader of the groundbreaking Recovery trial of Covid treatments at the University of Oxford, said the decision by the Medicines and Healthcare products Regulatory Agency (MHRA) was very welcome and amounted to a watershed moment in tackling the pandemic.

The MHRA said on Thursday that the drug developed by Ridgeback Biotherapeutics and Merck Sharp and Dohme (MSD), was safe and effective at reducing the risk of hospitalisation and death in people with mild to moderate Covid who are at increased risk of developing severe disease.

The health secretary, Sajid Javid, said the decision marked a “historic day”, adding “the UK is now the first country in the world to approve an antiviral that can be taken at home for Covid-19. This will be a gamechanger for the most vulnerable and the immunosuppressed, who will soon be able to receive the groundbreaking treatment.”

No 10 announced a deal last month to buy 480,000 doses of molnupiravir, also known as Lagevrio, after a clinical trial in the US showed a five-day course of the pills halved the risk of hospitalisation or death for at-risk patients. The first delivery is expected no earlier than mid-November.

Molnupiravir interferes with the virus’s ability to replicate, meaning it is less able to multiply and reach high enough levels in the respiratory system to cause severe disease. Because the drug is most effective when given in the early stages of infection, the MHRA recommends it is used as soon as possible after a positive test for Covid and within five days of symptoms appearing.

The drug is approved for people with mild to moderate Covid and at least one risk factor for developing severe illness, such as obesity, diabetes mellitus, heart disease, or being older than 60.

“Following a rigorous review of the data by our expert scientists and clinicians, we are satisfied that Lagevrio [molnupiravir] is safe and effective for those at risk of developing severe Covid-19 disease and have granted its approval,” said Dr June Raine, the MHRA chief executive.

“Lagevrio is another therapeutic to add to our armoury against Covid-19. It is also the world’s first approved antiviral for this disease that can be taken by mouth rather than administered intravenously. This is important because it means it can be administered outside of a hospital setting, before Covid-19 has progressed to a severe stage.”

Horby said: ”The approval of an oral antiviral for Covid-19 is a major scientific achievement and, to me, feels like a bit of a watershed. The MHRA are to be commended for, once again, moving very quickly to ensure the UK population gets access to drugs with proven benefit.

“We do, however, have some more work to do, such as ensuring access and affordability, avoiding drug resistance, and seeing if this drug can benefit other patient groups such as those hospitalised with Covid-19.”