Trump, 74, wore a mask as he waved from the back seat of a black SUV that crawled in a caravan of vehicles in front of the Walter Reed National Military Medical Center, while supporters waving Trump 2020 flags chanted "USA! USA!"
In a video posted on Twitter, Trump said he planned "a little surprise" for his fans gathered outside.
Trump has taken two doses of a five-day course of the intravenous antiviral drug Remdesivir, his doctors said at a briefing on Sunday morning, as well as the steroid dexamethasone, which is used in severe cases.Dr. Sean P. Conley acknowledged that Trump's blood oxygen levels had dropped in prior days and that he had run a high fever on Friday morning, admitting that the president's condition had been worse than previously disclosed. Conley said Trump was improving on Sunday.
The briefing came the day after contradictory messages from the White House caused widespread confusion about the president's condition.
Trump spent much of the year downplaying the risks of the COVID-19 pandemic, which has infected 7.4 million Americans, killed more than 209,000, and caused an economic downturn that has thrown millions out of work.
His illness has upended his re-election campaign as it seeks to fend off Democratic challenger Joe Biden in the final month of the race, and rattled financial markets. Several members of his inner circle have also tested positive for the disease, as well as three Republican members of the US Senate.
Two members of the White House residence staff tested positive for COVID-19 a few weeks ago, and Trump's "body man" aide, Nicholas Luna, has also tested positive, according to a source familiar with the situation.
The New Jersey Department of Health said on Sunday the White House had provided it with the names of more than 200 people who attended a Trump fundraiser at his golf club in Bedminster on Thursday, after the White House knew that adviser Hope Hicks had tested positive.
Asked what tests had revealed about the condition of Trump's lungs, Conley replied: "There's some expected findings, but nothing of any major clinical concern."
Conley's response suggests the X-rays revealed some signs of pneumonia, said Dr. Amesh Adalja, an infectious disease specialist at Johns Hopkins University.
"The expected finding is that he has evidence of pneumonia in the X-ray. If it was normal they would just say it is normal," Adalja said.
DEXAMETHASONE STEROID TREATMENT
Doctors said the president had a high fever on Friday morning but had not run a fever since Friday.
Dr. Brian Garibaldi said Trump was given the steroid dexamethasone in response to "transient low oxygen levels."
"He received his first dose of that yesterday and our plan is to continue that for the time being," Garibaldi said.
Dexamethasone is shown in studies to improve survival for patients hospitalized with severe or critical COVID-19 who need extra oxygen. But it should not be given in mild cases since it can limit the body's own ability to combat the virus, according to guidelines from the Infectious Disease Society of America.
Trump is also being given an experimental treatment, Regeneron's REGN-COV2, as well as zinc, Vitamin D, famotidine, melatonin and aspirin, his doctors have said.
"Our plan for today is to have him eat and drink, be up out of bed as much as possible, to be mobile," Garibaldi said.
Trump released a four-minute video on Saturday in which he said the "real test" of his condition will come over the next few days.
"Over the next period of a few days, I guess that's the real test, so we'll be seeing what happens over those next couple of days," Trump said into the camera, looking tired and wearing a jacket and open-necked shirt.
He tweeted thanks to his supporters on Sunday morning and afternoon.
"I really appreciate all of the fans and supporters outside of the hospital," Trump tweeted Sunday afternoon.
Differing assessments of Trump's health from administration officials on Saturday left it unclear how ill the president had become since he tested positive for the coronavirus on Thursday night.
Conley commented on the contradictory assessments on Sunday.
"I was trying to reflect an upbeat attitude of the team and the president about the course his illness has had," Conley told reporters on Sunday. "I didn't want to give any information that might steer the course of illness in another direction and in doing so, it came off that we were trying to hide something, which isn't necessarily true."
'AGGRESSIVE' CAMPAIGN CONTINUES
A Reuters/Ipsos poll published on Sunday found Biden had opened a 10-point lead over Trump nationally, slightly wider than it has been for the past two months. Some 65% of Americans said Trump likely would not have been infected had he taken the virus more seriously - a view that half of registered Republicans polled supported. Some 55% said they did not believe Trump had been telling the truth about the virus.
Trump's campaign vowed that Vice President Mike Pence, who would assume the presidency if Trump were unable to carry out his duties, would have an "aggressive" campaign schedule this week, as would Trump's three oldest children.
"We can't stay in our basement or shut down the economy indefinitely. We have to take it head-on," Trump campaign senior adviser Jason Miller said on ABC's "This Week" on Sunday.
Pence, who tested negative on Friday, is scheduled to debate Democratic vice presidential nominee Kamala Harris on Wednesday.
Biden, who largely avoided direct criticism of Trump during a campaign trip to Michigan on Friday, took a more aggressive tone on Saturday while speaking to a transit workers' union, even as he wished the president well.
"I'm in a little bit of a spot here, because I don't want to be attacking the president and the first lady now," Biden said, adding he hoped Trump and his wife, Melania, who also has the illness, make a full recovery.
But he quickly turned to Trump's response to the pandemic, calling it "unconscionable" and blasting Trump's comment in an interview this summer that "it is what it is" when asked about the death toll.