US President Joe Biden back in isolation after testing Covid+ again

US President Joe Biden has tested positive for COVID-19 again, slightly more than three days after he was cleared to exit coronavirus isolation, the White House said, in a rare case of "rebound" following treatment with an anti-viral drug.

White House physician Dr Kevin O'Connor said in a letter on Saturday that Biden "has experienced no re-emergence of symptoms, and continues to feel quite well." O'Connor said "there is no reason to reinitiate treatment at this time."

In accordance with Centres for Disease Control and Prevention guidelines, Biden will re-enter isolation for at least five days. He will isolate at the White House until he tests negative. The agency says most rebound cases remain mild and that severe disease during that period has not been reported.

Word of Biden's positive test came — he had been negative Friday morning — just two hours after the White House announced a presidential visit to Michigan this coming Tuesday to highlight the passage of a bill to promote domestic high-tech manufacturing. Biden had also been scheduled to visit his home in Wilmington, Delaware, on Sunday morning, where first lady Jill Biden has been staying while the president was positive. Both trips have been cancelled as Biden has returned to isolation.

Biden, 79, was treated with the anti-viral drug Paxlovid, and tested negative for the virus on Tuesday and Wednesday. He was then cleared to leave isolation while wearing a mask indoors. His positive tests put him among the minority of those prescribed the drug to experience a rebound case of the virus.

White House COVID-19 coordinator Dr Ashish Jha told reporters on July 25 that data "suggests that between 5 and 8 per cent of people have rebound" after Paxlovid treatment.

"Acknowledging the potential for so-called rebound COVID positivity observed in a small percentage of patients treated with Paxlovid, the President increased his tested cadence, to protect people around him and to assure early detection of any return of viral replication," O'Connor wrote in his letter.

O'Connor cited negative tests for Biden from Tuesday evening, Wednesday morning, Thursday morning and Friday morning, before Saturday morning's positive result by antigen testing. "This in fact represents rebound positivity," he wrote.

Both the Food and Drug Administration and Pfizer point out that 1 per cent to 2 per cent of people in Pfizer's original study on Paxlovid saw their virus levels rebound after 10 days. The rate was about the same among people taking the drug or dummy pills, "so it is unclear at this point that this is related to drug treatment," according to the FDA.

While Biden was testing negative, he returned to holding in-person indoor events and meetings with staff at the White House and was wearing a mask, in accordance with CDC guidelines. But the president removed his mask indoors when delivering remarks on Thursday and during a meeting with CEOs on the White House complex.

Asked why Biden appeared to be breaching CDC protocols, press secretary Karine Jean-Pierre said, "They were socially distanced. They were far enough apart. So we made it safe for them to be together, to be on that stage."

Also watch: Omicron likely to cause 3 times more reinfections than Delta: study

Regulators are still studying the prevalence and virulence of rebound cases, but the CDC in May warned doctors that it has been reported to occur within two days to eight days after initially testing negative for the virus.