A vaccine manufactured by Pfizer could be available by the end of the week, after anticipated approval by the Food and Drug Administration, Dr. Slaoui said on CNN’s “State of the Union” on Sunday. Alex M. Azar II, the secretary of health and human services, was just as optimistic.

“Really within days,” Mr. Azar said on “Fox News Sunday.” “Within 24 hours of F.D.A. green lighting with authorization, we’ll ship to all of the states and territories that we work with. And within hours, they can be vaccinating.”…

Health experts said the timeline sketched out by Dr. Slaoui and Mr. Azar was uncompromising and did not account for the possibility of delay during the many steps from vaccine manufacture to distribution at state and local levels, not to mention the hesitancy that many people might feel about taking a newly approved vaccine.

“To meet those kinds of aggressive timelines, all the stars would have to align,” said Dr. Peter J. Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine.