Gilead Sciences Inc's two clinical studies of its potential coronavirus treatment remdesivir will wind down by the end of May. The end of these trials effectively closes off a path of patient access to the antiviral medication, according to United States researchers involved in the studies.

The drug was given emergency use authorisation by the US Food and Drug Administration (FDA) on May 1, but hospitals are concerned about access. "We would like to see equitable and transparent distribution of this very precious resource," Dr Helen Boucher, chief of infectious diseases at Tufts Medical Center in Boston, told Reuters.

Gilead's studies - one on patients with severe COVID-19 and the other on patients with moderate cases of the disease - have enrolled around 8,000 subjects, according to FDA statistics. The trials are "open label", meaning they do not compare the treatment to a placebo, and participants know they are getting the drug.

Interest in Gilead's drug has been high given some promising early data and the lack of approved treatments or preventive vaccines for COVID-19, the disease caused by the novel coronavirus that has infected more than four and a half million people and killed more than 306,000 worldwide.

Preliminary results from a trial conducted by the US National Institutes of Health (NIH) showed that remdesivir cut hospital stays by 31 percent compared to a placebo. The NIH is now studying remdesivir alone compared to remdesivir in combination with Olumiant, an anti-inflammatory drug approved for rheumatoid arthritis and sold by Eli Lilly and Co.

Remdesivir is still available on a compassionate-use basis for pregnant women or children under the age of 18, but most COVID-19 patients will soon have access only under the emergency use authorisation.

"We participate in the Gilead clinical trials here at Tufts," Dr Boucher said. "We were notified that they will wind down ... no later than the end of May."

Gilead told Tufts it is transitioning to product distribution under the emergency use authorisation.

Researchers at Boston's Beth Israel Deaconess Medical Center have also heard that the two Gilead studies would stop enrolling patients by the end of the month.

Gilead, which has pledged to donate 1.5 million vials of remdesivir to fight the global pandemic, did not respond to requests for comment. The drug is administered by infusion in hospitals.

The Department of Health and Human Services on Saturday said Gilead had committed to supply US hospitals with around 607,000 vials of remdesivir - about 40 percent of the donation, or enough to treat at least 78,000 patients.

After doctors had questioned the transparency of the allocation process, the federal agency said state health departments would distribute the drug.

The Infectious Diseases Society of America on Thursday asked to meet with the administration of US President Donald Trump to discuss how remdesivir will be allocated.

Gilead has said early results from its first study showed that the drug improved outcomes for patients with severe COVID-19, and it plans to announce findings from its second study in moderate patients later this month.