President Joe Biden has tested positive again for COVID-19 but currently has no symptoms, according to a letter from White House physician Dr. Kevin O'Connor.
O'Connor said 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.
MORE: What to know about Paxlovid, the COVID-19 therapy that Biden is taking
"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."
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.