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On Thursday, President Donald Trump said he’d tested positive for COVID-19. On Friday he was flown to Walter Reed Medical Center out of what the White House called “an abundance of caution.” And on Saturday, the state of the president’s health remained uncertain: White House physician Sean Conley said Trump was “doing very well.” But White House Chief of Staff Mark Meadows reportedly said the president’s vitals had been “very concerning.”

During a Saturday press conference, Conley’s team said Trump’s symptoms were resolving and improving and that the president wasn’t on oxygen and wasn’t having difficulty breathing or walking. But the team wouldn’t clarify whether Trump had received any supplemental oxygen since testing positive. And both The New York Times and The Washington Post reported that Meadows said the next 48 hours will be critical.

Trump’s vitals over the last 24 hours were very concerning, Meadows said after the press conference, according to both news outlets, and he added that “we’re still not on a clear path to a full recovery.” The Times also reported that anonymous sources said Trump had had trouble breathing on Friday and had been given supplemental oxygen before being moved to Walter Reed. The Post later said a senior administration official had confirmed reports that the president was given oxygen at the White House on Friday before being taken to the hospital.

The White House didn’t respond to a request for comment. A tweet Saturday had Trump thanking the staff at Walter Reed and saying that with their help he was “feeling well.” And a later tweet, posted to Trump’s account Saturday evening, featured a four-minute video of Trump. “I’m starting to feel good,” he says in the clip. “You don’t know, 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.” It’s unclear when the video was recorded.

A multidisciplinary medical team is engaged in “state of the art monitoring” of the president and looking for any evidence of complications from the coronavirus or from therapies being used to treat Trump, Conley’s team said during its press conference. The team said Trump had received a first dose of the experimental drug remdesivir on Friday evening and that its plan is to continue a five-day course of that treatment.

Trump, who’d had a fever, a cough and congestion, will stay at the medical center until the team decides it’s safe for him to return to the White House, the doctors said Saturday, declining to give a hard date on when the president will be discharged. The team said days seven to 10 of COVID infections, the inflammatory phase, can be a concern and that the doctors will proceed carefully. As of Saturday morning, Trump had been fever free for over 24 hours, the team said, adding that it remains “cautiously optimistic.” First lady Melania Trump, also infected, is doing well and convalescing at home, the doctors said.

Others in Trump’s circle test positive

On Friday, just after arriving at the hospital, Trump tweeted a video filmed in the White House, thanking people for their support. “I’m going to Walter Reed Hospital, I think I’m doing very well but we’re going to make sure that things work out,” he said in the video. “The first lady is doing very well.” Later he sent a short tweet saying things were “Going welI, I think!”

Trump tweeted the news about his test results Thursday night, as did the first lady. “We are feeling good and I have postponed all upcoming engagements,” she wrote.

Earlier in the evening, the president, 74, revealed that he and the first lady, 50, had entered quarantine after learning that top White House aide Hope Hicks had tested positive for the coronavirus.

Friday, following the president’s transfer to Walter Reed, Kellyanne Conway, former senior adviser to the president, tweeted that she too had tested positive for the virus. “My symptoms are mild (light cough) and I’m feeling fine,” she wrote. “I have begun a quarantine process in consultation with physicians.” On Saturday, former New Jersey Gov. Chris Christie tweeted that he’d tested positive for the virus. Others in Trump’s orbit who’ve tested positive include Trump campaign manager Bill Stepien and Republican Sens. Mike Lee, of Utah; Thom Tillis, of North Carolina; and Ron Johnson, of Wisconsin, CBS News reported.

Vice president Mike Pence and his wife tested negative Friday, as did Trump’s daughter Ivanka Trump and son-in-law Jared Kushner. Democratic presidential nominee Joe Biden said Friday that he and his wife have tested negative.

Wash hands, wear mask, social distance

Earlier this week, the world passed a grim milestone when Johns Hopkins University reported that more than 1 million people had died as a result of the novel coronavirus. That revelation came less than a week after the number of deaths linked to COVID-19 in the US surpassed 200,000.

As the virus has spread across the world, health care professionals have warned that certain groups of people, including older adults and people with underlying medical conditions, are at a higher risk of developing a serious or even fatal illness if they’re infected with COVID-19.

Read more: COVID: When to get tested for the coronavirus and how long it will take to get results

The coronavirus is spread mainly through respiratory vapor, such as when someone sneezes or coughs into the air around you. Some experts have also suggested that the virus can linger in the air and that this may play a role in transmission.

Health officials continue to encourage people to wash their hands regularly, wear a mask when out in public, practice social distancing and avoid close contact with people who are sick.

People who’ve been in contact with someone who’s tested positive for COVID-19 should contact their doctor or local health care provider. The US Centers for Disease Control and Prevention recommends getting a coronavirus test for the following reasons: if you’re experiencing symptoms of COVID-19, if you’ve had close contact with someone — within 6 feet for at least 15 minutes — with a positive COVID-19 test, or if you’ve been asked by your health care provider to get a test.

Read more: Coronavirus symptoms: The full list, according to the CDC

Who’s been tested

The announcement of the president’s and first lady’s positive coronavirus test results came just hours after the revelation about Hicks’ results. The president indicated in an earlier tweet Thursday that he and the first lady had begun the quarantine process.

Hicks has accompanied Trump on several campaign trips in recent days, including to the presidential debate with Biden on Tuesday evening and a campaign rally in Minnesota on Wednesday.

Though the candidates tackled topics including race relations, climate change and the Supreme Court during Tuesday’s face-off, the coronavirus dominated much of the debate. Trump defended his decision to often appear in public without a facial covering, explaining that he wears a mask “when needed.”

“I think masks are OK,” Trump said, when asked by moderator Chris Wallace why he typically appears in public without wearing one. He pulled out a mask from his suit jacket to show he carried it with him.

See also: Face masks: Here are the best and worst materials for protecting against coronavirus

“I put a mask on, you know, when I think I need it. Tonight is an example, everybody has had a test,” Trump said. “I wear a mask when needed. When needed, I wear masks.”

On Friday, Biden said he and his wife had tested negative for COVID-19. Vice presidential candidate Kamala Harris and her husband also said they tested negative on Friday.

Vice President Mike Pence and second lady Karen Pence both tested negative for COVID-19 on Friday, said Pence’s press secretary, Devin O’Malley.

“As has been routine for months, Vice President Pence is tested for COVID-19 every day,” said O’Malley in a tweet. “This morning, Vice President Pence and the Second Lady tested negative for COVID-19. Vice President Pence remains in good health and wishes the Trumps well in their recovery.”

Read more: Coronavirus pandemic 7 months later: Everything we know right now

Trump’s daughter Ivanka Trump and son-in-law Jared Kushner, who both serve as senior White House advisers, also tested negative on Friday, according to White House spokeswoman Carolina Hurley.

Judd Deere, a White House spokesperson, said Trump “takes the health and safety of himself and everyone who works in support of him and the American people very seriously.”

“White House Operations collaborates with the physician to the president and the White House Military Office to ensure all plans and procedures incorporate current CDC guidance and best practices for limiting COVID-19 exposure to the greatest extent possible both on complex and when the president is traveling,” Deere continued in a statement sent to CNN.

The Trump campaign on Friday said that previously announced events involving the president will be changed to virtual events or temporarily postponed.

Taking experimental antibody cocktail

In an update Friday at 4 p.m. ET, Press Secretary McEnany tweeted another report from the president’s doctor revealing Trump received “a single 8 gram dose of Regeneron’s polyclonal antibody cocktail.” The president has also been taking vitamin D, zinc, melatonin, aspirin and the antihistamine/antacid famotidine.

“The president remains fatigued but in good spirits,” the White House physician said. “First Lady Melania Trump remains well with only a mild cough and headache.”

According to a Sept. 29 statement from Tarrytown, New York-based Regeneron, the experimental medication “reduced viral load and the time to alleviate symptoms in non-hospitalized patients with COVID-19″ during trials.

The REGN-COV2 cocktail — which combines the two monoclonal antibodies of REGN10933 and REGN10987 and was designed especially to treat SARS-CoV-2 — is also being tested for preventing infection in people who have been exposed to coronavirus patients, and in treating hospitalized patients.

“The greatest treatment benefit was in patients who had not mounted their own effective immune response, suggesting that REGN-COV2 could provide a therapeutic substitute for the naturally-occurring immune response,” said George D. Yancopoulos, M.D., Ph.D. and Regeneron chief scientific officer. “These patients were less likely to clear the virus on their own, and were at greater risk for prolonged symptoms.”

CNET’s Jessica Dolcourt contributed to this report.