Scientists have hailed the findings as a "major breakthrough" after trials showed dexamethasone reduced death rates by around a third among the most severely ill COVID-19 patients admitted to hospital.

Dexamethasone is used to reduce inflammation in other diseases.

The results, announced on Tuesday, suggest the drug should immediately become standard care in patients with severe cases of the pandemic disease, the researchers who led the trials said.

"This is a result that shows that if patients who have COVID-19 and are on ventilators or are on oxygen are given dexamethasone, it will save lives, and it will do so at a remarkably low cost," said Martin Landray, an Oxford University professor co-leading the trial.

"It's going to be very hard for any drug really to replace this, given that for less than STG50 ($A90) you can treat eight patients and save a life," he told reporters in an online briefing.

Co-lead investigator Peter Horby said dexamethasone was "the only drug that's so far shown to reduce mortality - and it reduces it significantly".

"It is a major breakthrough," he said.

"Dexamethasone is inexpensive, on the shelf, and can be used immediately to save lives worldwide."

There are currently no approved treatments or vaccines for COVID-19, the disease caused by the new coronavirus which has killed more than 431,000 globally.

The RECOVERY trial compared outcomes of around 2100 patients who were randomly assigned to get the steroid, with those of around 4300 patients who did not get it.

The results suggest that one death would be prevented by treatment with dexamethasone among every eight ventilated COVID-19 patients, Landray said.

One death would be prevented among every 25 COVID-19 patients that received the drug and are on oxygen.

Among patients with COVID-19 who did not require respiratory support, there was no benefit from treatment with dexamethasone.

"The survival benefit is clear and large in those patients who are sick enough to require oxygen treatment, so dexamethasone should now become standard of care in these patients," Horby said.

A COVID-19 expert at the Wellcome Trust global health charity said the findings would "transform the impact of the COVID-19 pandemic on lives and economies across the world".

"Countless lives will be saved globally," Nick Cammack said in a statement responding to the results.

The RECOVERY trial was launched in April as a randomised clinical trial to test a range of potential treatments for COVID-19, including low-dose dexamethasone and the malaria drug hydroxycholoroquine.

The hydroxychloroquine arm was halted earlier this month after Horby and Landray said results showed it was "useless" at treating COVID-19 patients.

Global cases of infection with the novel coronavirus have reached more than eight million, according to a Reuters tally, and more than 434,000 people have died after contracting the virus.

Australian Associated Press