US pharmaceutical giant Pfizer on Tuesday announced a deal to make its oral antiviral Covid-19 medication available more cheaply in poorer countries, if the promising pill passes trials and regulatory approval.

Pfizer, which also produces a Covid vaccine, said it had signed an agreement to sub-licence production of its Paxlovid pill to generic drug manufacturers, without receiving royalties.

The deal with the global Medicines Patent Pool (MPP) would therefore make the candidate drug available at a lower cost in 95 low- and middle-income countries covering around 53 percent of the world's population.

It follows a similar deal for US rival Merck & Co last month.

The drug is to be taken with the HIV medicine ritonavir.

While the search for vaccines has resulted in multiple products being approved for emergency use in the pandemic, the hunt for treatments for those who have already caught the disease has not been as fruitful.

Interim data from ongoing trials on the pill demonstrated an 89 percent reduction in the risk of Covid-19-related hospitalisation or death compared to a placebo, in non-hospitalised high-risk adults with Covid-19 within three days of symptom onset, said Pfizer.

Similar results were seen within five days of symptom onset, it added.

The Geneva-based MPP is a United Nations-backed international organisation that works to facilitate the development of medicines for low- and middle-income nations.

MPP executive director Charles Gore said: "This license is so important because if authorised or approved, this oral drug is particularly well-suited for low- and middle-income countries and could play a critical role in saving lives."

The drug "is to be taken together with ritonavir, an HIV medicine we know well, as we have had a license for it for many years, and we will be working with generic companies to ensure there is enough supply for both Covid-19 and HIV."

Potential sub-licensees have until December 6 to register an expression of interest.