WASHINGTON -- US President Joe Biden tweeted Thursday noon that he's "doing great" despite a COVID-19 infection.

"Thanks for your concern," Biden wrote alongside a picture of him working behind a desk in quarantine. "Keeping busy!"

His wife, Jill Biden, tweeted earlier that Biden told her "he's feeling fine," with "a few mild symptoms."

The US first lady, who is on the road in Detroit, Michigan, added that she tested negative for COVID-19 in the morning and would continue to follow public health guidance and wear a mask.

US Vice President Kamala Harris, who met with Biden on Tuesday, tested negative on Thursday. Her office said she would remain masked and continue her schedule as planned.

White House press secretary Karine Jean-Pierre announced Thursday morning that Biden, 79, had tested positive for COVID-19 and is "experiencing very mild symptoms."

Biden joined a growing list of officials in his administration who have contracted the virus, including Harris, who tested positive in April.

Fully vaccinated and twice boosted, Biden has begun taking Paxlovid, an antiviral therapy produced by Pfizer and given to patients with COVID-19, according to a statement from Jean-Pierre.

"He will isolate at the White House and will continue to carry out all of his duties fully during that time," Jean-Pierre added.

The White House also released a memorandum from Biden's physician, Dr. Kevin O'Connor, who wrote that the president was experiencing a runny nose, fatigue, and an "occasional dry cough" that began on Wednesday evening. O'Connor said he recommended Biden start on Paxlovid.

Biden "will continue to work in isolation until he tests negative," Jean-Pierre said. "Once he tests negative, he will return to in-person work."

The White House also said it would provide a daily update on Biden's status as he continues to carry out the full duties of the office while in isolation.

The announcement came hours before Biden was scheduled to leave for a pair of events in Pennsylvania. He returned home from an overseas trip over the weekend and visited Massachusetts on Wednesday for a speech on climate.

"Per standard protocol for any positive case at the White House, the White House Medical Unit will inform all close contacts of the President during the day today, including any members of Congress and any members of the press who interacted with the President during yesterday's travel," the White House said.

It also revealed Biden's last previous test for COVID-19 was Tuesday when he had a negative test result.

COVID-19 cases and hospitalizations are rising across the United States, with BA.5, the newest Omicron subvariant, becoming the predominant lineage circulating in the nation.

BA.5 is the most easily transmissible COVID-19 variant to date, able to evade previous immunity from infection and vaccination, researchers said.

"Vaccines remain our single-most important tool to protect people against serious illness, hospitalizations, and death," White House COVID-19 Response Coordinator Dr. Ashish Jha told reporters earlier this month. "And staying up to date is essential as we see BA.5 rise across the country."

The total number of COVID-19 cases in the United States exceeded 90 million on Thursday, with over 1 million related deaths, according to the latest data from the Johns Hopkins University.