WASHINGTON (AP) — First lady Jill Biden has tested positive for COVID-19 again in an apparent “rebound” case, after she tested negative for the virus over the weekend.

President Joe Biden, who spent three days with his wife at their Rehoboth Beach, Delaware, vacation home, continues to test negative, the White House said. He also suffered a rebound case earlier this month after an initial recovery from the coronavirus.

Jill Biden first tested positive for the virus on Aug. 15, when she and her husband were vacationing in Kiawah Island, South Carolina. She isolated in the beach town until she received two negative tests and was cleared to meet the president in Delaware on Sunday.

Biden’s deputy communications director Kelsey Donohue said the first lady “has experienced no reemergence of symptoms, and will remain in Delaware where she has reinitiated isolation procedures.” Donohue said the White House Medical Unit has notified individuals who were in close contact with the first lady.

Jill Biden, 71, and her husband, 79, have been twice-vaccinated and twice-boosted with the Pfizer COVID-19 vaccine. She had been prescribed the antiviral drug Paxlovid, which has proven to be highly effective at preventing serious disease and death among those at highest risk from COVID-19, but a minority of those prescribed the drug have experienced a rebound case of the virus a few days after their initial recovery.

The first lady has been in Rehoboth Beach since Sunday and will remain there while she continues to isolate.

The White House said the president was considered a close contact, and would wear a mask “for 10 days when indoors and in close proximity to others” in accordance with Centers for Disease Control and Prevention guidance. The White House will also increase the frequency of his COVID-19 testing.

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Associated Press writer Darlene Superville contributed to this report.