COVID-19 patients are supposed to remain in isolation for at least 10 days after their symptoms began to make sure they aren’t still contagious and don’t spread the virus to other people, according to guidelines from the Centers for Disease Control and Prevention.

“If his symptoms started Wednesday, that would imply that he was maximally infectious around the time of the debate,” Dr. Rochelle Walensky, chief of infectious diseases at Massachusetts General Hospital in Boston, said in an interview with NPR.

In that case, Saturday would be in line with the CDC’s 10-day recommendation. But that scenario would also raise questions about the president’s presence at the debate. If his symptoms began later, Saturday would be too soon, Walensky and others say.

“He is around other very important people. He should not put anyone in harm’s way,” Dr. Michael Mina, an infectious disease specialist at the Harvard T.H. Chan School of Public Health, wrote in an email.