Biden, 79, has a runny nose, fatigue and an occasional dry cough, symptoms which he began to experience late on Wednesday, White House physician Kevin O’Connor said in a note released on Thursday.

The president had begun taking the antiviral treatment Paxlovid, O’Connor said.

Fully vaccinated and twice boosted, Biden said he was “doing well” in a video posted on his Twitter account.

In the 21-second clip, he also said he was “getting a lot of work done” and would continue with his duties.

A photograph on his Twitter account showed him smiling, wearing a blazer and sitting at a desk with papers.

White House Covid-19 coordinator, Dr Ashish Jha, said Biden’s oxygen levels were normal and the president would isolate for five days and return to public events once he had a negative test.

Biden became ill at a time when his administration is grappling with soaring inflation and Russia’s land assault on Ukraine.

His popularity has dropped sharply over the past year.

A Reuters/Ipsos survey completed on Tuesday showed just 36 per cent of people in the US approve of his job performance.

His illness forced cancellation of a trip to Pennsylvania where Biden intended to lay out plans to ask Congress for $US37 billion ($A54 billion) for crime prevention programs.

The White House provided an unusually detailed account of the president’s morning activities, including a series of phone calls to political allies.

The Pfizer Inc antiviral drug Paxlovid that Biden is taking has been shown to reduce the risk of severe disease by nearly 90 per cent in high risk patients if given within the first five days of infection.

But Paxlovid has in some cases been associated with rebound infections, in which patients improve quickly and test negative after a five-day course of the drug, with symptoms returning days later.

Dr Bruce Farber, chief of infectious diseases at Northwell Health in New York, who is not treating the president, said Paxlovid is likely the only treatment Biden will get unless his symptoms worsen.

“Elderly people are more at risk for developing complications from COVID,” Farber said.

“It dramatically is lower if you’ve been vaccinated and doubly boosted, which he has been, so I anticipate he will do very well.”

At Biden’s last physical in November 2021, doctors reported that the president has atrial fibrillation, a common irregular heartbeat for which he takes Eliquis, a drug designed to prevent blood clots and reduce the risk of heart attacks and stroke.

Jha said Biden would stop taking Eliquis and the statin Crestor while on his Paxlovid treatment to avoid a negative interaction between the drugs.

Multiple members of Biden’s administration and other senior figures in Washington DC have tested positive for the coronavirus in recent months, including Vice President Kamala Harris and House of Representatives Speaker Nancy Pelosi, both of whom have since tested negative and resumed working.

Biden set up strict Covid-19 safety protocols at the White House, urged the public to take the virus seriously and campaigned for everyone to get fully vaccinated.

He is tested regularly for the disease and anyone who meets with him or travels with him is tested beforehand, the White House has said.

Biden had last tested negative on Tuesday.

He has stopped wearing a mask at public events in recent months, and the White House dropped its mask requirement ahead of his March 1 State of the Union Address.

Biden joins a roster of world leaders who have contracted Covid-19 since the pandemic started in early 2020.

A month before he lost the 2020 presidential election to Biden, Donald Trump contracted the virus.

He, his wife Melania and other White House staff contracted it after an event for Supreme Court Justice Amy Coney Barrett in September 2020.

Jean-Pierre said Biden had been speaking with his staff by phone and will continue taking part in meetings by phone and Zoom from the White House while he recovers.

He would resume in-person work once he tested negative, she said.

Administration officials have acknowledged that Biden’s regular contact with advisers and supporters could expose him to Covid-19.

On Wednesday he travelled to Rhode Island and Massachusetts for a speech on climate change.

Biden’s wife Jill tested negative on Thursday morning in Detroit and plans to maintain her full schedule of travel in Michigan and Georgia, according to her spokesman Michael LaRosa.

-Reuters