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U.S. President Joe Biden walks on the South Lawn of the White House to board Marine One, in Washington, U.S., February 10, 2022. Image Credit: Reuters

President Joe Biden tested positive for Covid-19 for a second consecutive day as he continues to deal with a rebound case of the disease, according to the White House.

Biden "will continue his strict isolation measures" given the positive test on Sunday morning, his physician Kevin O'Connor wrote in a letter released by the White House. The doctor said the second positive test was unsurprising.

Biden took Pfizer Inc.'s Paxlovid, a five-day treatment course that has been shown to sharply reduce symptoms, after first testing positive on July 21. He concluded the drug therapy Monday evening and tested negative each of the four days after that.

Pfizer Chief Scientific Officer Mikael Dolsten said on Thursday that the company was working with the Food and Drug Administration to finalize plans for a study of Paxlovid "retreatment" among those who experience rebound.