WASHINGTON  — President Joe Biden tested positive for COVID-19 for the second straight day, in what appears to be in a rare case of "rebound" following treatment with an anti-viral drug.

In a letter noting the positive test, Dr. Kevin O'Connor, the White House physician, said Sunday that the president "continues to feel well" and will keep on working from the executive residence while he isolates.

Biden tested positive on Saturday, requiring him to cancel travel and in-person events as he isolates for at least five days in accordance with Centers for Disease Control and Prevention guidelines.

After initially testing positive on July 21, Biden, 79, was treated with the anti-viral drug Paxlovid. He tested negative for the virus on this past Tuesday and Wednesday, clearing him to leave isolation while wearing a mask indoors.

Research suggests that a minority of those prescribed Paxlovid to experience a rebound case of the virus. The fact that a rebound rather than a reinfection possibly occurred is a positive sign for Biden's health once he's clear of the disease.

"The fact that the president has cleared his illness and doesn't have symptoms is a good sign and makes it less likely he will develop long COVID," said Dr. Albert Ho, an infectious disease specialist at Yale University's school of public health.