WASHINGTON - US President Joe Biden's COVID-19 symptoms continued to "improve significantly", his physician said on Sunday.

Biden's "predominant symptom now is sore throat", Dr. Kevin O'Connor wrote in a memo to the White House. "This is most likely a result of lymphoid activation as his body clears the virus, and is thus encouraging."

"His rhinorrhea, cough and body aches have diminished considerably," O'Connor said.

The White House announced on Thursday morning that Biden, 79, had tested positive for COVID-19. He is now quarantining at the White House and continues to carry out his duties while receiving treatment.

The causative agent for Biden's COVID-19 infection is "most likely" the BA.5 variant, preliminary sequencing results showed.

BA.5, an Omicron subvariant and the most transmissible COVID-19 mutant to date, is dominant in the United States, according to data from the Centers for Disease Control and Prevention.