US President Joe Biden said he was "doing well" after testing positive for Covid-19 on Thursday, and the White House said he was experiencing only mild symptoms.
Mr Biden, 79, is the second US president to catch Covid-19, after Donald Trump came down with the virus before the 2020 election.
Whereas Mr Trump caught Covid before vaccines were available and ended up in hospital for three days, Mr Biden is fully vaccinated with the Pfizer jab, has had two booster shots and is now taking the antiviral drug Paxlovid that can further shorten the disease's course and reduce the risk of hospital admission.
"I'm doing well, getting a lot of work done," Mr Biden said in a video message released on Twitter.
"Thanks for your concern and keep the faith."
Press Secretary Karine Jean-Pierre said Mr Biden was "experiencing very mild symptoms" and his doctor, Kevin O'Connor, said in a letter that Mr Biden had a runny nose and "fatigue, with an occasional dry cough, which started yesterday evening".
"The president is fully vaccinated and twice-boosted, so I anticipate that he will respond favourably, as most maximally protected patients do," Dr O'Connor said.
Mr Biden, the oldest serving American president, has undergone a gruelling travel schedule in recent days, including a trip to the Middle East last week.
"Folks, I'm doing great," Mr Biden tweeted earlier, noting that he was "keeping busy" after the White House said he would continue to work in isolation.
He also apologised to leaders in Pennsylvania after cancelling a trip there.
Ms Jean-Pierre said Mr Biden “will isolate at the White House and will continue to carry out all of his duties fully during that time".
"He has been in contact with members of the White House staff by phone this morning, and will participate in his planned meetings at the White House this morning via phone and Zoom from the residence," she said.
Until now, the president had managed to dodge the virus, even as members of his inner circle caught it.
Previous waves of Covid-19 have swept through Washington’s political class, infecting Vice President Kamala Harris, Cabinet members, White House staffers and politicians. Mr Biden has increasingly stepped up his travel schedule and resumed holding large indoor events where not everyone has been tested.
When administered within five days of symptoms appearing, Paxlovid, produced by Pfizer, has been proven to bring about a 90 per cent reduction in hospital admissions and deaths among patients most likely to suffer from severe disease, the Associated Press reported.
First lady Jill Biden tested negative on Thursday morning and continued with her schedule on her trip to Detroit, Michigan. Following guidance for close contacts by the Centres for Disease Control and Prevention, she will be wearing a mask on her public outings.
The White House says Vice President Kamala Harris also tested negative on Thursday. She met Mr Biden on Tuesday and will be masked on her outings as well.
In a speech to more than 2,600 attendees at the White House Correspondents’ Association dinner on April 30, Mr Biden acknowledged the risks of attending large events but said it was worthwhile.
“I know there are questions about whether we should gather here tonight because of Covid,” he said.
“Well, we’re here to show the country that we’re getting through this pandemic.”
The US is currently experiencing a rise in coronavirus cases and hospital admissions due to the introduction of a new Omicron subvariant that has become dominant in the country and is believed to be more infectious than other variants seen in the pandemic.
CDC data shows the country currently has a seven-day average of at least 126,000 daily cases, a level not registered since February. It is also recording more than 35,000 daily hospital admissions for Covid-19.
Daily deaths counted nationally remain low but are beginning to rise. More than 1 million people have died of Covid-19 in the US.