Dr. Anthony Fauci warned Sunday that even if a COVID-19 vaccine becomes available, it still may not yield herd immunity.

In an interview with the Aspen Institute on Sunday, Fauci — the U.S.'s top infectious disease expert — said he would settle for a vaccine that is between 70 and 75% effective.

"I doubt seriously that any vaccine will ever be a hundred percent protective. The best we've ever done is measles, which is 97 to 98% effective. That would be wonderful if we get there. I don't think we will," Fauci said. "I would settle for 70, 75% effective vaccine because that would bring you to that level of would-be herd immunity level."

But a CNN poll says one-third of Americans do not plan to get a vaccine if it becomes available. Fauci said Sunday that if those figures hold true and a potential COVID-19 vaccine is only 75% effective, the U.S. population would not reach the herd immunity threshold that would kill the virus.

"That's one of the reasons why we have to make sure we engage the community as we're doing now to get community people, to help us for people to understand that we are doing everything we can to show that it's safe and that it's effective," Fauci said. "And it's for the good of them as individuals and in society to take the vaccine."

Three coronavirus vaccines are expected to be studied in large-scale clinical trials in the next three months.

Herd immunity comes when a sufficient portion of a population is immune to an infectious disease. That can come either come through prior illness or vaccination, making the disease unlikely to spread further.