Centers for Disease Control and Prevention Director Dr. Rochelle Walensky testifies during a Senate Health, Education, Labor and Pensions Committee hearing on Tuesday, May 11, 2021.

New guidelines from the Centers for Disease Control say fully vaccinated people can now safely participate in most indoor and outdoor activities without wearing a mask or social distancing.

"If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic. We have all longed for this moment when we can get back to some sense of normalcy," Dr. Rochelle Walensky, the director of the CDC said during a White House briefing, Thursday.

President Biden called the news a "great milestone." He credited the massive campaign to quickly vaccinate the American public.

To date, more than 154 million Americans have received at least one dose of a COVID-19 vaccine - about 47 percent of the population. Dr. Walensky says the new guidance about masks is based on science. Evidence shows the vaccine is strongly protective, even against mutations of the virus.

Still, there are some exceptions to the CDC's new recommendations. Masks are still required for anyone riding on buses, trains, or planes, and in hospitals.

Experts acknowledge these guidelines are based on the honor system and that there is no failsafe way to prove who is vaccinated and who is not.

"Any time you see someone without a mask, the risk is to the unvaccinated individuals so people can choose to take that risk. But science and good data suggest that the smart move is to get vaccinated," said Dr. Jen Ashton, ABC's Medical Correspondent.

According to a new AP-NORC poll, a wide variety of Americans from different backgrounds have questions and hesitations about the vaccines. A large number of those who remain unvaccinated in the U.S. say they would need to hear their concerns addressed by a credible source before feeling comfortable with the shot.

Kizzmekia Corbett is hoping to be that person, especially for the Black community.

"If I'm going to be a scientist and I'm going to try to create things that help people and benefit human health, then making sure that I translate that to people who are most concerned about it seems to be appropriate," Corbett told the Associated Press.

She helped lead the development of the Moderna vaccine and now spends hours giving plain-spoken answers to counter misinformation.

For example, Corbett told AP that while the clinical trials may have seemed fast to outsiders, scientists have been working on vaccines for earlier cousins of COVID-19 for the last six years.

COVID-19 deaths in the U.S. are now at the lowest level in 10 months, a milestone health experts say would not have been impossible without the vaccines.