WASHINGTON (NewsNation Now/WFLA) — The Centers for Disease Control and Prevention says fully-vaccinated Americans who live in areas with “substantial and high” transmission should wear masks in indoor public spaces in the wake of rising COVID-19 cases.

CDC Director Rochelle Walensky said they had reviewed new data in recent days that showed fully-vaccinated individuals could spread the delta variant more easily than previous versions of COVID-19.

“This new science is worrisome and unfortunately warrants an update to our recommendations,” she said on a conference call with reporters.

The CDC’s website has a map that shows counties with substantial and high transmission. That includes a combined 63.11% of counties in the U.S.

According to the CDC, the entire state of Florida is considered a high transmission area with the state accounting for a fifth of the nation’s new coronavirus infections last week, more than any other state.

“The vast majority of infection is happening with unvaccinated individuals,” Walensky said. She added the concern was vaccinated people may unknowingly introduce the disease to at-risk family or friends.

She said a vaccinated person’s risk of developing symptoms when exposed to the delta variant is reduced 7-fold, and the risk for hospitalization is reduced 20-fold.

The announcement reverses a decision made by the CDC just two months ago. The guidance specified that fully-vaccinated people did not have to wear masks outdoors in crowds and in most indoor settings. The guidance still called for wearing masks in crowded indoor settings like buses, planes, hospitals, prisons and homeless shelters.

The news comes as Gov. DeSantis continues to oppose mask mandates at public schools as Florida leads the nation in new coronavirus cases. DeSantis has been firmly opposed to lockdown restrictions, mask mandates and vaccine passports, signing a bill into law that prevents businesses to ask for proof of vaccination and local governments from imposing COVID restrictions.

Reuters and the Associated Press contributed to this report.