Joe Biden tested positive for Covid-19 for the second time and is returning to isolation, according to his White House doctor, who attributed the result to “rebound” positives following the US president’s treatment.

Following four days of negative tests, Biden “tested positive late Saturday morning, by antigen testing,” and “will reinitiate strict isolation procedures,” presidential physician Kevin O’Connor said in a memo.

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“This in fact represents ‘rebound’ positivity,” O’Connor was referencing to a case in which people treated with the medicine Paxlovid, such as Biden, eliminate the virus yet test positive after finishing their course.

“The president has experienced no re-emergence of symptoms and continues to feel quite well. This being the case, there is no reason to reinitiate treatment at this time,” O’ Connor added.

Biden appeared to be attempting to downplay the situation in a tweet.

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“Folks, today I tested positive for Covid again. This happens with a small minority of folks,” the 79-year-old president wrote. “I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon.”

According to the White House, Biden postponed planned travels to Delaware and Michigan.

The second positive test occurred just three days after O’Connor stated that Biden had tested negative and did not need to isolate, as he had done since receiving the first positive result on July 21.

In contrast to his predecessor, Donald Trump, who sometimes insulted individuals who wore masks, Biden has been visibly meticulous about following Covid protocols.

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“The President continues to be very specifically conscientious to protect any of the Executive Residence, White House, Secret Service and other staff whose duties require any (albeit socially distanced) proximity to him,” O’Connor assured.

As the oldest US president in history (he turns 80 in November), Biden’s health is constantly scrutinised.

On Wednesday, he was released from his five-day Covid seclusion, looking energised as he told applauding aides that his speedy recovery should encourage Americans to take advantage of free vaccines and treatments.

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He compared his seeming speedy recovery to Trump’s more serious brush with the sickness in October 2020, when vaccines were not available.

“When my predecessor got Covid, he had to get helicoptered to Walter Reed Medical Center,” Biden stated. “He was severely ill. Thankfully, he recovered. When I got Covid, I worked from upstairs of the White House.”

He also stated that getting fully vaccinated, getting preventative testing, and then using the Paxlovid therapeutic reduces deaths and is free of charge.

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“You don’t need to be president to get these tools,” he added.

After relieving Biden from his first round of Covid, O’Connor advised that the president would wear a mask for 10 days and continue to test routinely in case of a “rebound.”

According to O’Connor, Biden is in good health overall. He has been fully immunised and has had two coronavirus booster doses.