Updated at 8:07 p.m. ETPresident Trump briefly left Walter Reed National Military Medical Center on Sunday evening to wave to supporters gathered outside.A masked Trump was seen waving to supporters from a black SUV. Other images showed Secret Service personnel in the vehicle with personal protective equipment on.Right before the drive-by, the president tweeted a video in which he thanked the doctors and staff at Walter Reed for treating him for COVID-19 and said he was about to "make a little surprise visit" outside to greet supporters."We're going to pay a little surprise to some of the great patriots that we have out on the street, and they've been out there for a long time and they've got Trump flags and they love our country," Trump said. In the video, Trump said his experience with the coronavirus has been "a very interesting journey." "I learned a lot about COVID. I learned it by really going to school. This is the real school. This isn't the, 'let's read the book' school," he said. Trump has been briefed extensively about the pandemic by members of his White House coronavirus task force and other advisers."I get it, and I understand it, and it's a very interesting thing I'm going to be letting you know about it," Trump said, thanking the nurses and doctors for their work. Some experts swiftly characterized Trump's drive-by greeting as reckless. Dr. James Phillips, chief of disaster medicine at George Washington University, lambasted the move as being made for "political theater.""That Presidential SUV is not only bulletproof, but hermetically sealed against chemical attack. The risk of COVID-19 transmission inside is as high as it gets outside of medical procedures. The irresponsibility is astounding. My thoughts are with the Secret Service forced to play," he wrote on Twitter. White House spokesman Judd Deere said the excursion had been cleared by the medical team beforehand. "Appropriate precautions were taken in the execution of this movement to protect the president and all those supporting it, including PPE," he said in a statement.The president very rarely makes any movement in public without a press pool. In this case, the pool was not informed of the president leaving Walter Reed."President Trump took a short, last-minute motorcade ride to wave to his supporters outside and has now returned to the Presidential Suite inside Walter Reed," Deere said in a separate statement after the trip."It is outrageous for the president to have left the hospital — even briefly — amid a health crisis without a protective pool present to ensure that the American people know where their president is and how he is doing," the White House Correspondents' Association said in a statement. "Now more than ever, the American public deserves independent coverage of the president so they can be reliably informed about his health."His brief visit outside came hours after Trump's medical team told reporters that the president experienced two drops in his oxygen levels over the course of his COVID-19 diagnosis, is being treated with a steroid, and could be discharged from Walter Reed as early as Monday. Copyright 2020 NPR. To see more, visit https://www.npr.org.