Biden 'continues to feel well' after testing positive for COVID-19 again: Doctor

President Joe Biden has tested positive again for COVID-19, according to a letter from White House physician Dr. Kevin O'Connor, who said on Sunday that Biden "continues to feel well" and that "unsurprisingly" his test results are still positive.

O'Connor wrote in a memo released by the White House that Biden's antigen test came back positive late Saturday morning after he tested negative Tuesday evening, Wednesday morning, Thursday morning and Friday morning.

"This is in fact rebound positivity," O'Connor wrote, referring to a relapse of COVID-19 some patients experience after taking Paxlovid, an antiviral treatment for people with mild to moderate symptoms who are considered a high risk for severe illness.

While uncommon, some patients can test positive again after finishing a course of Paxlovid. But doctors say this doesn't mean the drug isn't working. Overall, high-risk patients who take Paxlovid still have a dramatically lower risk of being hospitalized.

"The President has experienced no reemergence of symptoms, and continues to feel quite well," O'Connor wrote. "This being the case there is no reason to reinitiate treatment at this time, but we will obviously continue close observation."

In an update on Biden's health that was shared by the White House on Sunday, O'Connor wrote that he will "continue strict isolation measures" from the White House residence. Biden "continues to be very specifically conscientious" of any staff who are required to be in proximity to him, such as Secret Service agents, O'Connor wrote.

Biden had six close contacts before testing positive for COVID again and none of those people have tested positive, according to a White House official.

The president acknowledged his positive test in a Twitter post on Saturday afternoon, noting he is going to reinitiate isolation measures.

"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 reemerged from the White House residence for the first time on Wednesday after working for five days in isolation following his first positive test on July 21.

In a speech delivered from the Rose Garden, Biden praised vaccines and therapeutics as the keys to his mild case of the virus.

His symptoms included a runny nose, cough, sore throat, a slight fever and body aches. He took the standard five-day course of Paxlovid, and also used Tylenol and an albuterol inhaler as needed.

"We got through COVID with no fear, I got through it with no fear, a very mild discomfort because of these essential, life-saving tools," Biden said from the Rose Garden. "You don't need to be president to get these tools used for your defense."

Before news of his positive COVID-19 test, Biden was set to travel to his home in Wilmington, Delaware, on Sunday -- which would have marked his first trip since his initial diagnosis.

Biden was also scheduled go to Michigan on Tuesday to discuss the recently passed "Chips and Science" Act, which provides billions of dollars in government subsidies for U.S. production of semiconductors and computer chips used in everything from automobiles to medical devices.

"The President will isolate at the White House until he tests negative. The President will not travel to Wilmington or Michigan as planned," the White House said in a statement on Saturday.

Biden is fully vaccinated and has received two booster shots, but at the age of 79 is considered to be at a higher risk for severe illness. Paxlovid is used to lower the risk of severe illness for people who are considered high-risk, including those 65 and older and those with underlying medical conditions.

Dr. Anthony Fauci, the White House's chief medical adviser, also experienced a rebound case after taking Paxlovid.

Fauci, 81, tested negative for three consecutive days before getting another positive test. He then started a second course of Paxlovid when symptoms returned, which he described as "much worse than in the first go around."

White House press secretary Karine Jean-Pierre said in a Twitter post Saturday that Biden is "asymptomatic, feeling fine, and working in isolation from the [White House] Residence to protect others."

The Centers for Disease Control and Prevention doesn't suggest or recommend any testing within 90 days of a positive case unless new symptoms are developed.

The CDC has also noted that a COVID-19 rebound has been reported to occur between 2 and 8 days after initial recovery. Biden tested negative for three days before receiving a positive result again on the antigen test.

It's not clear if a rebound in positivity means someone is contagious. The CDC said transmission of infection during COVID-19 rebound has been described, "however, it remains unknown whether the likelihood of transmission during rebound differs from the likelihood of transmission during the initial infection."

ABC News' Justin Gomez contributed to this report.