President Joe Biden said Thursday he was "doing great" after testing positive for Covid at the age of 79, vowing to continue to work while isolating at the White House with only mild symptoms.
Biden's personal physician Kevin O'Connor wrote in an official note that the US president was experiencing fatigue, a runny nose and an occasional dry cough, starting on Wednesday evening.
In a series of posts crafted to show he remains at the country's helm, Biden tweeted a picture of himself working at his desk saying "I am doing great," and a short video clip filmed on a White House balcony.
"Hey folks, I guess you heard this morning I tested positive for Covid," said the president, wearing a blazer, no tie and the hint of a smile.
"Symptoms are mild, I really appreciate all your inquiries and concern... it's going to be OK."
Announcing the diagnosis, the White House stressed that the president was fully vaccinated and twice boosted. It said Biden was taking Pfizer's Paxlovid pill, an antiviral used to minimize the severity of Covid-19.
"Consistent with CDC (Centers for Disease Control and Prevention) guidelines, he will isolate at the White House and will continue to carry out all of his duties fully during that time," it added.
The president tested positive in a rapid antigen test Thursday morning before the result was confirmed by a PCR (polymerase chain reaction) test.
He is reported to be in good general health, but as the oldest US president ever elected his age heightens concern over the impact of Covid.
A physical exam in November last year found Biden "fit for duty."
But facing a punishing workload, Biden -- like other presidents before him -- is showing the strain: his walk is stiffer, his hair thinner, and his speech less fluent than when he took office.
Politically Biden is also in a tough phrase of his presidency, facing November midterm elections that are forecast to be painful for his Democratic Party, as well as declining personal approval ratings.
Higher risk for elderly
The president had planned to spend more time on the ground in the United States in the coming weeks after a period of intense overseas travel, including a NATO summit in Spain and a controversial trip to Saudi Arabia.
He is now due to stay at the White House for several days.
The CDC, the leading US government health agency, says that older adults are more likely to get severely sick from Covid-19, with risk increasing with age.
The highly-transmissible Omicron BA.5 subvariant is currently fueling a new Covid wave in the United States -- where hospitalizations have doubled in recent months.
The country is seeing some 130,000 new reported cases daily, but the figure is thought to be substantially underestimated since home tests are not typically reported to authorities.
On taking office, Biden made a point of abiding by strict Covid protocols, holding socially distanced meetings or Zoom gatherings, and wearing a mask to public events -- in sharp contrast to his predecessor Donald Trump.
While other members of Biden's family and many close advisors contracted the virus, the president had remained Covid free until now.
Vice President Kamala Harris, who caught Covid back in April, tested negative following Biden's diagnosis, the administration said.
Biden last tested negative on Tuesday, and his officials pledged to provide a daily health update as proof of transparency, as criticism grew that he was not tested on Wednesday.
White House Covid-19 Response Coordinator Ashish Jha, briefing the media, sought to use Biden's case to promote vaccination jabs.
"Because the president is fully vaccinated, double boosted, his risk of serious illness is dramatically lower," Jha said.
Return from Middle East
Briefing reporters, Biden's spokeswoman Karine Jean-Pierre dismissed questions about where and how he caught the virus, saying "I don't think that matters."
Biden returned from the Middle East in the early hours of Sunday, and on Wednesday travelled to Massachusetts to deliver a speech on climate change.
"I talked to him just a few minutes ago. He's doing fine, he's feeling good," First Lady Jill Biden, who tested negative, told reporters Thursday morning during a visit to a school in Michigan.
Good wishes poured in from US politicians across the country's sharp divide as well as from abroad.
"Wishing you... a swift recovery from #COVID19. Stay strong!" tweeted World Health Organization chief Tedros Ghebreyesus.
"Wishing President Biden a speedy recovery from COVID," said leading US Republican Lindsey Graham.
Then-president Trump tested positive for coronavirus in October 2020 -- in the middle of the bitterly fought election race he eventually lost to Biden.
Trump, who was 74 at the time, was treated in a clinic for three days before returning to the White House. (AFP)