(CNN) President Donald Trump is being treated at Walter Reed National Military Medical Center for Covid-19 and has begun receiving an anti-viral drug, his doctor announced Friday night.
In a memo shared by White House press secretary Kayleigh McEnany, White House physician Navy Cmdr. Dr. Sean Conley wrote Trump is doing "very well."
"This evening I'm happy to report that the President is doing very well. He is not requiring any supplemental oxygen, but in consultation with specialists we have elected to initiate Remdesivir therapy," Conley wrote, referring to a drug that has been shown to shorten coronavirus patients' hospital stays. "He has completed his first dose and is resting comfortably."
The news comes after a chaotic day marked by Trump's announcement early Friday morning that he and first lady Melania Trump had tested positive for Covid-19, followed by the sudden news Friday afternoon that he was being taken to Walter Reed at Conley's recommendation.
Emerging from the White House residence at 6:16 p.m. ET for his first public appearance since announcing 16 hours earlier he had tested positive for coronavirus, Trump walked under his own power to his waiting helicopter and displayed no major outward signs of illness.
Wearing a navy blue suit, a blue silk necktie and a dark face mask, Trump waved to the media and gave a thumbs up, but did not stop to talk. Chief of staff Mark Meadows, also wearing a mask, followed him aboard.
Trump landed at the hospital a short time later, saluting his military aides before climbing into his limousine for a brief ride to the hospital's main building. His hand could be seen waving as he sped past assembled media.
After his arrival, the President posted an 18-second video to his Twitter account, seeking to reassure the American people he is doing "very well" after his coronavirus diagnosis.
"I want to thank everybody for the tremendous support. I'm going to Walter Reed Hospital. I think I'm doing very well. We're going to make sure that things work out. The first lady is doing very well. So thank you very much, I appreciate it. I will never forget it. Thank you," he said.
He sent a tweet late Friday night, his first communication from the hospital, saying, "Going welI, I think! Thank you to all. LOVE!!!"
His upbeat attitude did not reflect the inherent severity of the situation. It remains extremely rare for a president to overnight in hospital, given the extensive medical facilities available at the White House.
Trump himself was said to be spooked after he announced he tested positive early Friday, and has become increasingly alarmed by his diagnosis as he developed symptoms like a fever overnight, according to a person familiar with his reaction.
Several people in Trump's close orbit have tested now positive for coronavirus, including campaign manager Bill Stepien, former White House counselor Kellyanne Conway and adviser Hope Hicks.
A Trump adviser said there is reason for concern about the President's health.
"This is serious," the adviser said. The adviser went on to describe Trump as very tired, very fatigued and having some trouble breathing.
A source familiar with the situation said White House officials have serious concerns about Trump's health and a top administration official told CNN that Trump is "OK for now, but our fear is that things can change quick."
A third source said the President's condition is worse than first lady Melania Trump's.
Still, there has been no transfer of power to Vice President Mike Pence, said White House spokeswoman Alyssa Farah. "The President is in charge," she said.
A White House official on Friday evening stressed there is no reason for the public to be alarmed about Trump's condition.
The official acknowledged that Trump is dealing with some symptoms of the virus and is "fatigued." But, according to the official, the President's condition is not deteriorating. The official said there are plans to keep the public updated on Trump's health over the coming days.
The President is taking the situation "very seriously," the official added.
The White House had continued to insist the President "remains in good spirits, has mild symptoms, and has been working throughout the day" in the hours before he traveled to Walter Reed.
"Out of an abundance of caution, and at the recommendation of his physician and medical experts, the President will be working from the presidential offices at Walter Reed for the next few days," McEnany said. "President Trump appreciates the outpouring of support for both he and the First Lady."
An administration official said Pence was working from his residence at the Naval Observatory and remains in good health.
Trump has had a fever since Friday morning, a person familiar with the matter said, though an official said the fever remains consistent with the White House's description of "mild symptoms."
Earlier in the afternoon, Trump's physician wrote in a memo he "remains fatigued but in good spirits."
"He's being evaluated by a team of experts, and together we'll be making recommendations to the President and First Lady in regards to next best steps," Conley wrote.
He said Trump had been administered a Regeneron polyclonal antibody cocktail and has been taking zinc, vitamin D, famotidine, melatonin and a daily aspirin.
The decision to give Trump an experimental monoclonal antibody cocktail illustrates how concerned the White House may be about his Covid-19 diagnosis, Dr. Jonathan Reiner, CNN medical analyst and professor at George Washington University, told CNN on Friday.
"The big news there is that he got an experimental drug. He got a drug not approved by the FDA," said Reiner, who has treated former Vice President Dick Cheney.
The memo also said Melania Trump remains well with only a mild cough and headache, and the remainder of the first family is well and have tested negative.
The President disclosed in the early morning hours on Friday that he and the first lady had tested positive for coronavirus.
The development threw the country's leadership in turmoil and lent new uncertainty to the unfolding presidential race.
Moments before news broke that Trump was heading to Walter Reed, Democratic presidential nominee Joe Biden said he was suspending negative advertising for the time being.
The Gang of Eight -- top leaders of Congress and top lawmakers on the House and Senate intelligence committees -- have not been briefed on Trump's health, according to two sources familiar.
The group typically gets briefed on classified intelligence and national security matters, the sources added.
Two officials from two previous administrations tell CNN that the "presidential offices" consist of a suite of rooms on the sprawling campus of Walter Reed, including a sitting room, a conference room and a hospital bedroom.
But the officials, one Democrat and one Republican, both said the explanation in McEnany's statement stood out to them as curious. The offices are not dedicated for presidential use only, but the suite of offices is a place where presidents can go while they are on the medical campus.
This story has been updated with additional developments.