WASHINGTON (AP) — The White House says President Joe Biden tested positive for COVID-19 again Saturday, slightly more than three days after he was cleared to exit coronavirus isolation.

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It's a rare case of “rebound” following treatment with an anti-viral drug.

White House physician Dr. Kevin O’Connor says 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.

Answers to 25 common COVID-19 vaccine questions

Vaccinations for COVID-19 began being administered in the U.S. on Dec. 14, 2020. The quick rollout came a little more than a year after the virus was first identified in November 2019. The impressive speed with which vaccines were developed has also left a lot of people with a lot of questions. The questions range from the practical—how will I get vaccinated?—to the scientific—how do these vaccines even work?

Keep reading to discover answers to 25 common COVID-19 vaccine questions.