The Centers for Disease Control and Prevention revised its masking guidelines on Tuesday afternoon, urging Americans living in areas with “substantial” and “high” transmission of Covid-19 to wear a protective face covering indoors regardless of their vaccination status. CNN reports that nearly two-thirds of all US counties are likely impacted.

The CDC also recommended that all students, teachers, and staff wear masks when schools re-open this fall.

“The delta variant is showing its ability every day to outsmart us,” CDC Director Rochelle Walensky said on Tuesday. She pointed to new data that suggests even vaccinated patients can spread the disease in rare circumstances.

CDC Director Dr. Rochelle Walensky notes:

“In rare occasions, some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others.” pic.twitter.com/4L5A55WOJ4

— The Recount (@therecount) July 27, 2021

CNN outlines what areas are likely impacted:

In two states, Arkansas and Louisiana, every county is currently listed as having “high” levels of community transmission of Covid-19, according to CDC data. Several other states — including Missouri, Mississippi and Alabama — also have “high” transmission in nearly every county. In Florida, every county was listed with high transmission on Monday; as of Tuesday morning, one — Glades County — has substantial transmission.

More from Bloomberg:

Former CDC Director Julie Gerberding said the agency is responding to the change in transmission patterns as the pandemic evolves. “That is exactly what we need them to do — follow the science,” she said.

“Nobody wants to bring the delta variant home,” Gerberding said. “That is why masking and other common sense measures make sense until vaccination coverage improves and transmission once again slows.”

Walensky reiterated that the COVID-19 vaccines offer protection against severe illness, hospitalization and death. But the delta variant’s transmissibility caused the CDC to amend its guidelines.

“This new science is worrisome and unfortunately warrants an update to our recommendations,” she said. 

The announcement, which reverses lax mask guidelines established in May, comes as COVID-19 cases have quadrupled this month. “The vast majority” of new cases, according to Walensky, are among the unvaccinated.

Earlier in the day, White House press secretary Jen Psaki addressed the new CDC guidelines:

We continue to be at war with a virus, an evolving pandemic. Our responsibility here is to always lead with the science, and always lead with the advice of health and medical experts and we’re going to continue to provide information to all of you about how to protect yourself and save your lives. We’re not saying that wearing a mask is convenient, or people feel like it, but we are telling you that that is the way to protect yourself protect your loved ones and that’s why the CDC is issuing this guidance.