Joe Biden Tests Positive For COVID-19 Again

President Joe Biden tested positive for COVID-19 again on Saturday, three days after a negative test allowed him to resume work as usual from the Oval Office.

The presidential physician, Dr. Kevin O’Connor, wrote in a memo that the antigen test represents a “rebound” case of the coronavirus. He attributed it to the Paxlovid treatment Biden was prescribed; the pills are meant to lower the chance of requiring hospitalization for high-risk COVID-19 patients.

“This happens with a small minority of folks,” Biden said in a tweet, adding that he has no symptoms but plans to isolate himself in the White House residence.

White House press secretary Karine Jean-Pierre added that the president was “feeling fine.”

O’Connor stated that “there is no reason to reinitiate treatment at this time, but we will obviously continue close observation.” His office will provide updates as needed, he wrote.

The White House announced that Biden, who is fully vaccinated and has received two booster shots, initially came down with the virus on Thursday, July 21, experiencing “very mild” symptoms.

Pfizer received an emergency use authorization for its Paxlovid treatment in December, with the Food and Drug Administration permitting its use in patients age 12 and up who weigh at least 88 pounds. Clinical trials showed that the drug reduced the risk of hospitalization and death by 89%, but it must be taken within five days of developing symptoms.

A dose of the drug comes in the form of three pills: Two are nirmatrelvir, which inhibits viral replication, and one is ritonavir, which helps the nirmatrelvir work for a longer period of time.

Given the chance of “so-called rebound COVID positivity observed in a small percentage of patients treated with Paxlovid,” O’Connor said, “the president increased his testing cadence, both to protect people around him and to assure early detection of any return of viral replication.”

O’Connor added: “After testing negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning, the president tested positive late Saturday morning by antigen testing.”

Biden’s Sunday trip to Delaware and a trip next week to Michigan, planned to promote the CHIPS Act, have both been canceled.

This article originally appeared on HuffPost and has been updated.