President Joe Biden tested positive for COVID-19 Thursday morning, the White House announced.

The 79-year-old is fully vaccinated and twice boosted and is experiencing very mild symptoms, Press Secretary Karine Jean-Pierre said in a statement. He has already begun a course of the antiviral Paxlovid and has started an isolation period in the White House, consistent with guidelines from the Centers for Disease Control and Prevention.

According to a letter released from the president's physician, Kevin O'Connor, Biden tested positive via an antigen test this morning, a result that has already been confirmed by a PCR test. Prior to this morning, he was most recently tested on Tuesday and had tested negative. Now, Biden is experiencing mild symptoms of a runny nose, fatigue, and an occasional dry cough, which began yesterday, the letter said.

Noting that Biden has begun a course of Paxlovid, O'Connor added, "I anticipate that he will respond favorably, as most maximally protected patients do." The antiviral has proven highly effective at preventing severe disease in those at high risk, such as older adults.

Jean-Pierre said that Biden will remain in isolation until he tests negative, breaking from the CDC guidelines. The CDC notably does not require people to test negative before leaving isolation. Rather, the agency advises that people can leave isolation after five days if they've been fever-free for 24 hours, though they should take precautions, such as mask-wearing, for a full 10 days after testing positive.

In isolation, Biden is expected to continue carrying out his full duties. He will participate in all of his planned meetings today via phone or Zoom. "Out of an abundance of transparency, the White House will provide a daily update on the president’s status," Jean-Pierre added.

The positive test comes just days after Biden returned from a trip overseas, where he had close contact with leaders in Israel and the Middle East.

This story is breaking and this post may be updated.