WASHINGTON (AP) — President Joe Biden tested positive for COVID-19 again Saturday, slightly more than three days after he was cleared to exit coronavirus isolation, the White House said, in a rare case of “rebound” following treatment with an anti-viral drug.

White House physician Dr. Kevin O’Connor said in a letter that Biden “has experienced no reemergence of symptoms, and continues to feel quite well.”

In accordance with Centers for Disease Control and Prevention guidelines, Biden will reenter isolation for least five days. The agency says most rebound cases remain mild and that severe disease during that period has not been reported.