First lady Jill Biden has tested positive for COVID-19 again in an apparent "rebound" case, after she initially tested negative for the virus over the weekend.

President Joe Biden, three days with his wife at their Rehoboth Beach, Del. 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 virus.

Jill Biden first tested positive for the virus on Aug. 15, when she and her husband were vacationing in Kiawah Island, S.C. 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 she "has experienced no reemergence of symptoms, and will remain in Delaware where she has reinitiated isolation procedures." She added: "The White House Medical Unit has conducted contact tracing and close contacts have been notified."

Jill Biden, 71, like her husband, has 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 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.