Doctors not involved in treating President Donald Trump for COVID-19 said the fact that he has been started on dexamethasone - a generic steroid widely used in other diseases to reduce inflammation - is the strongest evidence yet that his case may be severe.

Trump's medical team on Sunday said the president was started on the steroid after experiencing low oxygen levels, but his condition was improving and he could be discharged from the hospital on Monday.

"What I heard in the news conference description suggested the President has more severe illness than the generally upbeat picture painted," said Dr. Daniel McQuillen, an infectious disease specialist at Lahey Hospital & Medical Center in Burlington, Massachusetts.

The Infectious Disease Society of America says dexamethasone is beneficial in people with critical or severe COVID-19 who require extra oxygen. But studies show that the drug is not helpful – and may even be harmful – in people with a milder case of the illness.

Given the patient is 74 years old, overweight and possibly at high risk of complications, "they were aggressive at the beginning," said Dr. Stuart Cohen, chief of infectious disease at California's UC Davis Health.

He and other doctors who have been treating COVID-19 patients for months said Trump, who surprised cheering supporters outside the hospital by riding past in a motorcade Sunday evening, could still be discharged from the hospital. Trump returned to Walter Reed National Military Medical Center after the short trip. 

"He's not going to go to a home where there's no medical care. There's basically a hospital in the White House," said Dr. Walid Gellad, professor of medicine at University of Pittsburgh.

Trump was flown to the hospital on Friday hours after announcing that he had tested positive for coronavirus infection. While at the White House, the president was given an infusion of an experimental antibody treatment from Regeneron Pharmaceuticals that is being studied for early infections. On Saturday, the president was started on a five-day course of intravenous antiviral drug remdesivir, which is sold by Gilead Sciences.

Doctors have said that both of these drugs makes sense early in the course of illness to prevent it from getting worse, but dexamethasone is generally reserved for people whose condition has deteriorated.

"We give dexamethasone to patients who require supplemental oxygen," said Dr. Amesh Adalja, an infectious disease specialist at Johns Hopkins University.

If Trump no longer requires supplemental oxygen and is able to return to his normal activities, his doctors could discharge him from the hospital, he said.

"The biggest question would be is there a risk of deterioration, or is he on a good trajectory?" Dr. Adalja said.

COVID-19 is often characterized as having two phases - the viral infection itself and in some cases an overreaction of the body's immune system that can cause organ damage. "People sort of putter along for up to a week ...then everything goes downhill very quickly," Dr. Cohen said. "It is always hard to predict who that is going to happen in."

Doctors said COVID-19 patients who have had a good response to treatment can leave the hospital relatively quickly, but they will still need to be closely monitored.

"Some people with COVID-19 develop worsening symptoms, shortness of breath and other complications about a week after they first develop symptoms," said Dr. Rajesh Gandhi, an infectious disease physician at Massachusetts General Hospital in Boston.

Dr. David Battinelli, chief medical officer at New York's Northwell Health said "it's entirely plausible" that Trump could get discharged on Monday, but cautioned that a full recovery would take time.

"It would be very unlikely for him to be out and about, and on the campaign trail in less than 14 days," he said.