Biden has reportedly tested positive for COVID yet again. Is he experiencing ‘Paxlovid rebound’?

US President Joe Biden has tested positive for COVID yet again—this after testing negative Tuesday evening and Wednesday morning, when he exited isolation.

The president will resume isolation despite not experiencing new symptoms, he said in a Saturday tweet.

“Folks, today I tested positive for COVID again,” 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, 79, is one of millions of Americans who tested positive for BA.5, as confirmed last week by his personal physician, Dr. Ashish Jha. His initial positive test came July 21, after a week of traveling across the Middle East. He tested negative for the virus on Tuesday night and again on Wednesday morning of this past week, when he ended his isolation, according to the Associated Press.

What is Paxlovid?

The president was treated with the COVID-19 antiviral Paxlovid the day he was diagnosed, according to multiple news outlets. It’s a pill approved for the treatment of COVID-19 in high-risk adults and has emergency-use authorization from the US Food and Drug Administration.

The drug is known for “rebound” cases, referred to by the US Centers for Disease Control and Prevention as “a brief return of symptoms” that “may be part of the natural history of SARS-CoV-2 (the virus that causes COVID -19) infection in some persons, independent of treatment with Paxlovid and regardless of vaccination status.”

In May the CDC issued a health advisory about such rebounds, saying there was no evidence that additional treatment is needed for rebound cases.

When diagnosed with COVID in June, chief presidential medical adviser Dr. Anthony Fauci was prescribed the antiviral due to his advanced age, which put him at greater risk for a severe outcome from COVID despite being fully vaccinated and twice boosted.

Fauci later said he had experienced a Paxlovid rebound, but clarified that the drug was important to his recovery.

“After I finished the five days of Paxlovid, I reverted to negative on an antigen test for three days in a row,” Fauci, 81, said during an event at Foreign Policy’s Global Health ForumBloomberg reported. “And then on the fourth day, just to be absolutely certain, I tested myself again.”

“I reverted back to positive,” he said.

But Fauci told the New York Times that Paxlovid kept him away from the hospital, and stopped his infection from becoming more severe initially. “Paxlovid did what it was supposed to do,” he said in an interview with the outlet.

Fauci began a second course of Paxlovid when symptoms emerged “much worse than the first go-around,” he said.

In June, Pfizer, which manufactures the medication, announced that it would stop adding new participants to a trial of the drug among COVID patients at low risk of hospitalization and death. The study failed to demonstrate that the drug reduced symptoms, or hospitalizations and deaths in a statistically significant way, according to Bloomberg.