President Donald Trump will stay at Walter Reed military hospital for an indefinite number of days, his doctor said on Saturday, undergoing a five-day antiviral drug regimen for COVID-19 that indicated his condition might be more serious than had been publicly disclosed. On Thursday, he received another therapy that is still in early phases of testing.

At a news conference on Saturday, Dr. Sean Conley, Trump’s physician, painted a rosy picture of the president’s condition, saying he was doing well, fever-free and not in need of supplemental oxygen.

But immediately afterward, White House Chief of Staff Mark Meadows contradicted that assessment and said Trump’s vital signs in the last 48 hours “were very concerning” and that the next two days would be critical.

Medical experts who have been following the sketchy, conflicting details released by the White House and Walter Reed were divided about the experimental treatments Trump, 74, has already received.

But they generally welcomed his close monitoring at the nation’s military medical complex, noting his risk factors — that he’s an older man who is slightly obese — for developing a more serious form of the disease.

“Transferring him to Walter Reed was the right thing,” said Dr. Carlos del Rio, professor of medicine at Emory University in Atlanta. “Because any of us who have seen patients with COVID, especially patients at his age, can say that one minute they seem fine and the next minute not.”

The doctors at the news conference stressed the critical period of time — about 7 to 10 days after infection — when a patient’s condition can take a turn for the worse. Some people respond to an infection with an overly exuberant immune response that can worsen their illness and even prove fatal.

Infectious disease experts said it was hard to comment on Trump’s condition or evaluate his treatment with the scant and contradictory information that had been released publicly, but some said they were worried.

“It is a very weird situation,” said Dr. Andre Kalil, an infectious disease specialist at the University of Nebraska. “Is he being overtreated or undertreated? We don’t know.”

One of the therapies being given to Trump, a combination of monoclonal antibodies made by Regeneron, is in the earliest phase of testing, and there is only a recent news release from the company on its effectiveness.

In one trial, the treatment is being given to recently exposed or infected people, especially those who may not be able to rouse a potent immune response on their own. By age and gender, Trump falls into the subgroup of people whose immune system may not be able to fend off the virus.

Some experts are optimistic that the Regeneron drug works and support giving patients a therapy that directly attacks the virus. Monoclonal antibodies in general are pretty safe, experts said, and similar ones have been used to treat patients with HIV and Ebola.

Other experts were more skeptical.

“In my mind these antibodies are experimental,” said Kalil, a principal investigator for the federal study showing the other treatment, remdesivir, that is being given to Trump, sped recovery in hospitalized COVID patients.