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

CDC Director Rochelle Walensky said they 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.

All people in schools should also wear masks, regardless of vaccination.

“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.

This comes following the director of the CDC, Dr. Rochelle Walensky, saying just two weeks ago that the COVID-19 outbreak was becoming “a pandemic of the unvaccinated.”

For months COVID cases, deaths and hospitalizations were falling steadily, but those trends began to change at the beginning of the summer as a mutated and more transmissible version of the coronavirus, the delta variant, began to spread widely, especially in areas with lower vaccination rates.

“I know 18 months through this pandemic not only are people tired, they’re frustrated,” Walensky said on the call. “We have mental health challenges in this country, we have a lot of continued sickness and death in this country. Our health systems are in some places, being overrun for what is preventable. And I know in the context of all of that it is not a welcomed piece of news that masking is going to be a part of people’s lives who have already been vaccinated.”

Los Angeles County announced earlier this month it will reinstate an indoor mask mandate to combat the surge in coronavirus cases. Chicago recently revived its COVID-19 travel restrictions for states seeing significant case spikes.

About 163.2 million people, or 49.1 % of U.S. adults, have been fully inoculated with COVID-19 vaccines made by Pfizer Inc/ BioNTech SE, Moderna Inc. and Johnson & Johnson, according to CDC data. 188.7 million people, or 56.8% million adult Americans have received at least one dose of a COVID-19 vaccine.

“Public health experts, scientific experts, medical experts, when we when has shown them these [delta variant] data, have universally said that this required action,” Walensky said. “I felt that when I saw the data myself.”

Reuters and the Associated Press contributed to this report.