Dr. Anthony Fauci, the nation's top infectious disease expert, says he's "cautiously optimistic" about the possibility of a vaccine as research into it goes into full throttle.

Former FDA Commissioner Scott Gottlieb believes that COVID-19 case count will explode over the next several weeks.

Fauci told the CEO of the American Medical Association says that the U.S. has six vaccine candidates in the works, with five of them in phase three trials and two of them fully enrolled.

Having a vaccine at all, though, isn't a cure. Fauci cautions there are two key questions at play.

"First of all, how effective would the vaccine be? And, as importantly, how many individuals are going to opt to take the vaccines?" says Fauci.

If there is a reasonably effective vaccine and a substantial number of people take it, Fauci says, "I think we're going to be going in the right direction towards approaching some degree of normality as we head into 2021 and the second, third, and fourth quarter of 2021."