President Joe Biden has tested positive again for COVID-19, according to a letter from White House physician Dr. Kevin O'Connor. Biden currently has no symptoms, he said.

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.