By Jeff Mason

(Reuters) – The White House tried to reassure Americans on Friday that President Donald Trump was still working from isolation, after his bombshell announcement that he had caught the coronavirus threw the administration and presidential race into uncertainty.

Trump, who has played down the threat of the coronavirus pandemic from the outset, said he and his wife Melania had tested positive for the deadly virus and were going into quarantine.

Wall Street tumbled at news of one of the biggest health scares involving an American president for decades, with the S&P 500 plunging more than %1.5 at the opening.

“The president is not incapacitated. He is actually working from the residence,” a senior White House official said.

Vice President Mike Pence and his wife tested negative, a Pence spokesman said. The White House official said Pence would work from his own residence and his staff was being kept separated from Trump’s staff “out of an abundance of caution”.

But the official acknowledged that the president’s illness would force him to cancel travel plans with just 31 days left to go until the presidential election. Polls show Trump trailing his Democratic opponent, former Vice President Joe Biden.

“We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!” Trump said in a tweet early on Friday morning.

Biden said on Twitter that he and his wife Jill wished Trump and the first lady a speedy recovery. “We will continue to pray for the health and safety of the president and his family,” he said.

Trump, 74, is at high risk because of his age weight. He has remained in good health during his time in office but is not known to exercise regularly or to follow a healthy diet.

Trump advisers acknowledged that they would have to rip up their plans for the final weeks of the election campaign. Trump has held in-person rallies with supporters who mainly decline to wear masks, and has mocked Biden for avoiding such events.

“It’s so early to kind of know what’s going to happen but clearly it changes the dynamic from us being able to travel and show enormous energy and support from the rallies, which has been part of our calculation,” a Trump adviser said.

Trump’s positive test also means that others at the highest levels of the U.S. government have been exposed and may have to quarantine, too. A White House official said early on Friday that contact tracing was under way.

Trump’s physician, Sean Conley, said he expected the president to carry out his duties “without disruption” while he recovers.

“The President and First Lady are both well at this time, and they plan to remain at home within the White House during their convalescence,” Conley wrote in a memo that was distributed to the press.

Leaders around the world wished Trump a speedy recovery.

On Thursday night, shortly after Trump predicted the pandemic’s end was in sight, news broke that Hope Hicks, a top adviser and trusted aide, had tested positive for the virus. Hicks traveled with the president on Air Force One on Tuesday and Wednesday.

Trump flew to New Jersey after White House officials learned of Hicks’s symptoms, and attended a fundraiser at his golf club and delivered a speech. Trump was in close contact with other people, including campaign supporters, at a roundtable event.

CHANGE IN THE RACE

The president’s condition is likely to bring the pandemic back to center stage in the race, after the death of Supreme Court Justice Ruth Bader Ginsburg shifted the campaign’s focus.

Trump, who has been criticized for questioning the efficacy of wearing a face covering, produced a mask from his pocket in his first debate against Biden on Tuesday and said, “I wear masks when needed. When needed, I wear masks.”

The White House has had previous coronavirus scares. Pence’s spokeswoman, Katie Miller, tested positive earlier this year and suffered symptoms before recovering. A military valet also came down with the virus.

But the White House lowered its precautions as Trump sought to project a return-to-normalcy message this summer. Temperature checks of everyone entering the White House complex stopped, and while coronavirus tests continued for people who came into close proximity to Trump, others on campus were not tested.

(Reporting by Jeff Mason; Additional reporting by Hideyuki Sano and Aishwarya Nair; Writing by Jeff Mason; Editing by William Mallard, Michael Perry and Peter Graff)