Washington, July 25 : US President Joe Biden's Covid-19 symptoms have continued to "improve significantly", his physician said in a memo to the White House. Biden's "predominant symptom now is sore throat", Xinhua news agency quoted Dr. Kevin O'Connor as saying in the memo on Sunday. "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 the morning of July 21 that Biden, 79, had tested positive for Covid-19. He is currently quarantining at the White House and continues to carry out his duties while receiving treatment. The causative agent for the President's Covid infection is "most likely" the BA.5 variant, preliminary sequencing results showed. BA.5, an Omicron sub-variant and the most transmissible Covid mutant to date, is dominant in the US, according to data from the Centers for Disease Control and Prevention. /IANS