Biden has experienced “no recurrence of symptoms and continues to feel reasonably well” and therefore will not resume treatment, the White House said. O’Connor said the president tested negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning before testing positive on Saturday morning.
“However, given his positive antigen test, he will begin strict isolation procedures again,” the doctor noted. Biden stopped self-isolating on Wednesday after testing negative in back-to-back antigen tests, celebrating his comeback in remarks from the White House Rose Garden.
A White House official said contact tracing efforts were underway Saturday after Biden tested positive for Covid-19.
The president had resumed events at the White House after testing negative this week, although he was seen wearing a mask more regularly. White House officials said they were working to ensure social distancing at public events he held at the White House.
When Biden first tested positive last week, the White House said he had 17 close contacts, none of whom ultimately tested positive.
First lady Jill Biden remains in Delaware, where she has been since the president first tested positive for Covid-19, a White House official told CNN. She did not return to the White House.
The first lady was expected to join the president on his trip to Wilmington on Sunday.
Biden is fully vaccinated and has received two boosters. He received his first two doses of the Pfizer/BioNTech Covid-19 vaccine before its inauguration in January 2021, his first booster in September and his second booster in March.
White House officials had previously suggested a rebound Covid case was unlikely, based on case reports across the country. However, Biden continued to be tested and monitored.
Infectious disease experts have called on the government to study rebound cases more systematically, saying the phenomenon needs to be better characterized to understand who is most at risk and whether the standard five-day treatment with the drug needs to be extended to prevent this. Studies have shown that people can transmit the infection to others when rebounding, which is another reason to understand it better.
Pfizer, the company that makes Paxlovid, said its studies show that rebounds are rare and occur in people who take the drug as well as in those who have taken a placebo pill. Because investigators noted the phenomenon in both groups, the company does not believe it is drug-related.
This story has been updated with additional developments.
CNN’s Kate Bennett, Brenda Goodman, Virginia Langmaid and Jamie Gumbrecht contributed to this report.