CHICAGO (NewsNation Now) — Currently, nearly 58 percent of Americans age 12 and up are fully vaccinated, but health experts say it’s not enough to “crush the outbreak.”

With the highly contagious delta variant spreading across the country, the Centers for Disease Control and Prevention has brought back guidelines recommending masks even for vaccinated Americans in areas where the spread of the virus is serious.

Dr. Anthony Fauci, the nation’s top infectious disease expert, says the U.S. can “expect more pain and suffering” ahead and that most new infections are among the unvaccinated.

“If you look at the acceleration of the number of cases, the seven-day average has gone up substantially. We say it over and over again, and it’s the truth — we have 100 million people in this country who are eligible to be vaccinated who are not getting vaccinated,” Fauci said Sunday on ABC’s This Week with George Stephanopoulos.

The CDC says Flordia has set a new record with more than 21,600 new COVID-19 cases reported Friday.

Florida’s state health department reported coronavirus cases jumped 50% last week — continuing a six-week surge —bringing numbers back to where they were in January, just before vaccines became widely available.

In Missouri, COVID-19 cases tripled in July. In California, two of San Francisco’s largest hospitals say more than 250 staff members are infected with COVID-19.    

Officials at San Francisco General Hospital and UCSF Medical say most of the infections are breakthrough cases of people who are fully vaccinated. The cases are among physicians, nurses, and administrative staff of the hospital’s 7,500 employees. They said nearly 100-percent of the cases were traced back to community spread.

Dr. Fauci says breakthrough infections are mostly mild or without symptoms whereas unvaccinated people are much more vulnerable and likely to get a severe case that might lead to hospitalization or even death.

Despite the current surge in cases, Dr. Fauci says we are in a better situation than we were at the beginning of the pandemic. 

“I don’t think we’re going to see lockdowns. I think we have enough of the percentage of people in the country, not enough to crush the outbreak, but I believe enough to not allow us to get into the situation we were in last winter,” Fauci said Sunday on ABC’s This Week with George Stephanopoulos.

However, as a result of the climbing cases and the rising threat from the delta variant, doctors say more Americans are rolling up their sleeves and getting their shots.

“That’s what desperately needs to happen if we are going to get this delta variant put back in its place because right now it’s having a pretty big party in the middle of the country,” National Institutes of Health Directory Dr. Francis Collins said Sunday on CNN’s State of the Union.

As the end of summer nears and kids prepare to return to the classroom, the CDC has also recommended that all teachers, staff, students, and visitors wear their masks indoors at school regardless of vaccination status.