Nearly a third of all Americans are fully vaccinated against the coronavirus.

It's why many states have recently announced new timelines to fully reopen.

Data from the Centers for Disease Control and Prevention (CDC) shows more than 56% of U.S. adults have received at least one dose of the COVID-19 vaccine.

In order to reach herd immunity, health leaders say that number needs to be around 70 to 85%. 

But some experts worry the demand may not be there. 

"What I really worry about is that those people who are already on the fence don't get vaccinated, we don't reach herd immunity come the fall, and in the winter, because coronaviruses are a big respiratory viruses we have a big resurgence, maybe we have variants coming in from other countries," said Dr. Leana Wen.

Doctors are worried younger adults aren't getting vaccinated. A Quinnipiac University poll found that 36% of adults under the age of 35 say they don't plan to get a shot.

This story was originally published by Robin Dich and Alex Livingston at Newsy.