Dr. Anthony Fauci, the nation’s top infectious disease expert, said Sunday that he’s “cautiously optimistic” that a coronavirus vaccine will be available by the start of 2021, but warned that the U.S. would likely not reach herd immunity if a substantial portion of the population refused to take it.

Fauci said during an interview with CNN that aired as part of the Aspen Ideas Festival that he would settle for a vaccine that was 70-75 percent effective, noting that it would “bring you to herd immunity level.” But he said that it was “unlikely” the U.S. would be able to quell the outbreak if 30 percent of the population refused to take the vaccine when asked about recent survey results.

“That’s one of the reasons why we have to make sure we engage the community as we’re doing now to get … people to understand that we are doing everything we can to show that it’s safe and effective and that it’s good [for] them as individuals and society to take the vaccine,” Fauci said. “We have a lot of work to do.”

Read more at The Hill.