US President Joe Biden is “doing well” and “getting a lot of work done” after testing positive to COVID for the first time and isolating in the White House while he stays on the job.

The 79-year-old, the oldest person to serve as US president, is considered high risk because of his age but is also double vaccinated and twice boosted.

Mr Biden has been taking the antiviral medication Paxlovid, which is authorised for high-risk patients and works by helping stop the virus multiplying in the body while the immune system kicks in.

The White House said Mr Biden was experiencing “very mild symptoms” including a runny nose, fatigue and occasional dry cough which he began to experience late on Wednesday (local time).

The President tested positive on Thursday local time.

“Consistent with CDC guidelines, he will isolate at the White House and will continue to carry out all of his duties fully during that time,” press secretary Karine Jean-Pierre said in a statement.

Ms Jean-Pierre said Mr Biden had spoken to his staff by phone and would continue taking part in meetings by phone and video-call from the White House while he recovered.

Joe Biden tweeted a photo of himself working in isolation. Photo: Twitter

Mr Biden tweeted a picture from isolation in which he was smiling, wearing a blazer and sitting at a desk with papers.

“Folks, I’m doing great. Thanks for your concern. Just called Senator Casey, Congressman Cartwright, and Mayor Cognetti (and my Scranton cousins!) to send my regrets for missing our event today. Keeping busy!” he wrote on Twitter.

In another video post he thanked the public for its concern and assured people “it’s going to be OK”.

An update from me: pic.twitter.com/L2oCR0uUTu

— President Biden (@POTUS) July 21, 2022

The White House provided an unusually detailed account of Mr Biden’s morning activities, including phone calls to political allies.

The President’s wife, Dr Jill Biden, told media she had tested negative and would continue with her schedule of travel in Michigan and Georgia.

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

Pfizer’s antiviral drug that Mr 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 but then days later symptoms return.

Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, experienced a rebound infection four days after ending his first course of Paxlovid. He wa given a second round of the drug, he said this week.

Mr Biden’s White House predecessor, Donald Trump, also contracted COVID and was hospitalised for three nights in October 2020.

Mr Biden is fully vaccinated and got his second booster shot in March.

The risk of an unvaccinated person aged 65 and older being hospitalised after contracting COVID is 10 times that of someone who has been vaccinated.

Unlike Mr Trump, Mr Biden consistently wore a mask when in public when case counts were high and before being vaccinated.

But Mr Biden 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.

Multiple members of Mr 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 them have since tested negative and resumed working.

Cases in the US are up more than 25 per cent in the past month, according to CDC data, as the rapidly spreading BA.5 subvariant has taken hold.

Evading the immune protection afforded either by vaccination or prior infection, BA.5 has been the dominant subvariant in the US since at least early July and has driven a surge of new infections globally.

-with AAP