Joe Biden has tested positive for Covid-19 for a second time and is returning to isolation, his White House doctor said Saturday, attributing the result to “rebound” positivity from treatment the US president received.

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

“This in fact represents ‘rebound’ positivity,” O’Connor said, referring to a situation in which patients treated with the drug Paxlovid — as Biden was — clear the virus but test positive after completing 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,” he added.

In a tweet, Biden seemed to seek to minimize the situation.

“Folks, today I tested positive for Covid again. This happens with a small minority of folks,” he 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.”

Biden canceled planned trips to Delaware and Michigan, the White House said.

‘Specifically conscientious’

The second positive test came just three days after O’Connor said Biden had tested negative and no longer needed to isolate, which he had been doing since receiving a first positive result on July 21.

Biden has for the most part been conspicuously careful about observing Covid protocols — in contrast to his predecessor Donald Trump, who sometimes mocked those who wore masks.

“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 said.

As the oldest US president in history — he will turn 80 in November — Biden’s health receives constant attention.

On Wednesday, he had ended his earlier five-day Covid isolation, appearing energetic as he told cheering aides that his quick recovery should inspire Americans to take advantage of free vaccines and treatments.

He contrasted his seemingly quick recovery to Trump’s more serious bout with the disease in October 2020, before vaccines were available.

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

He added that being fully vaccinated, taking preventative tests, then using the Paxlovid therapeutic prevents deaths and is available at no cost.

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

O’Connor had warned after clearing Biden from his first round of Covid that the president would wear a mask for 10 days when around others and continue to test regularly in case of a “rebound.”

O’Connor says Biden is generally in good health. He has been fully vaccinated and received two booster shots against the coronavirus.