Researchers said had that drug been used to treat patients in the UK from the start of the pandemic, up to 5000 lives could have been saved.

Since then many hospitals around the world adopted dexamethasone as part of their usual care for severely ill patients with Covid-19.

McArthur said they'd now confirmed hydrocortisone was just as effective.

"Going forward I think it will become a very common treatment in ICU."

Their findings meant clinicians now had options, especially if there were drug supply issues.

Both drugs were widely available in New Zealand hospitals, and cheap, McArthur said, also making it a huge benefit in poorer countries with high numbers of Covid-19 patients.

"An oral course of dexamethasone costs less than $1, or if intravenous is needed a course of hydrocortisone is about $150.

"So it is not expensive and is applicable anywhere in the world.

The international study pooled data from seven randomised clinical trials from 12 countries involving 1703 critically ill patients with Covid-19 from February 26 to June 9.

The New Zealand study was an extension to a wider international study, REMAP-CAP, that's been recruiting patients with severe community-acquired pneumonia – the main cause of death from Covid-19 – for the past three years.

In April, the trial was awarded additional funding by the Health Research Council and Ministry of Health, as it adapted to focus on Covid-19.

In the international analysis involving 1703 patients, the odds of death within four weeks was much lower among patients who received corticosteroids, compared with those who received usual care or placebo.

Of those patients randomly provided corticosteroids there were 222 deaths and 456 survivors, compared to 425 deaths and 600 survivors of those randomised to usual care or placebo.

The treatment was "very straightforward", and involved tablets taken orally daily, or intravenously if required, McArthur said.

The steroids worked by "dampening down" the body's immune response to Covid-19.

"We have learned from people particularly unwell with Covid-19, there can be an overly-active immune response, which can actually damage organs, particularly the lungs.

"We already have a steroid circulating in our blood, cortisol, which is a hormone and part of our stress response. What we do by adding these steroids, is increasing that cortisol effect, balancing the reaction, and allowing the natural healing process to proceed."

The New Zealand arm of the trial only involved six patients, which reflected the low numbers of cases here, but there was a lot of "operational support", McArthur said.

The ongoing trial would now be testing a range of other treatments to further improve the survival rate from Covid-19.

Convalescent plasma, anti-coagulation and anti-platelet treatments are planned to be added to the study in New Zealand soon.

HRC chief executive Professor Sunny Collings said trials like these were critical to answer life-saving questions during this pandemic.

"Thanks to RECOVERY, REMAP-CAP and the other trials in this meta-analysis, we now know that corticosteroids have an effect that's consistent across multiple trials."

Ministry of Health chief science adviser Professor Ian Town said the impact of Covid-19 on lung function had been a "worrying feature", and he welcomed the new findings the corticosteroids could help address it.

"We have known for a long time that steroids can help damp down the pulmonary inflammation in a variety of acute situations, so it is very encouraging to see a strong evidence base for their use in Covid-19."