Even fully vaccinated people should wear masks in public indoor places due to the highly contagious Delta variant, according to updated guidance from the CDC.

travelers wearing face masks
Credit: Stefani Reynolds/Bloomberg via Getty

The Centers for Disease Control and Prevention updated its mask guidance Tuesday, saying that in areas where COVID-19 is surging, all people — even those who have been fully vaccinated — should wear masks indoors.

"In recent days I have seen new scientific data from recent outbreak investigations showing that that Delta variant behaves uniquely differently from past strains of the virus that cause Covid-19," CDC Director Dr. Rochelle Walensky told reporters at a news briefing. "This new science is worrisome and unfortunately warrants an update to our recommendation."

The announcement marks a reversal of the organization's guidance from May 13 that only unvaccinated individuals needed to wear face coverings while in public.

Wolensky also said that everyone in K-12 schools should wear masks indoors when they return this fall.

"In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public, indoor settings to help prevent the spread of the Delta variant, and protect others. This includes schools," Walensky said. The CDC recommends that everyone grade schools wear masks indoors, "including teachers, staff, students and visitors, regardless of vaccination status."

Earlier this year, the CDC had stated that in most cases, Americans who are fully vaccinated against COVID-19 could be indoors or outdoors without masks or social distancing.

At the time, the announcement marked a major shift in public health guidance after a year of living with the pandemic. The CDC cited strong protection from the vaccines and the steep decline in new COVID-19 cases in that announcement, which was before the delta variant of COVID had taken hold.

Now, the updated rules come as the delta variant is gaining traction throughout the country.

White House chief medical advisor Dr. Anthony Fauci previously said that the CDC was determining whether or not to update its face-covering guidance for vaccinated individuals on Sunday.

Noting that it was "under active consideration," Fauci, 80, told CNN, "It's a dynamic situation. It's a work in progress, it evolves like in so many other areas of the pandemic. You've got to look at the data."

Guidance from the CDC comes as only a recommendation, CNBC reported. States and local officials will be in charge of whether or not they choose to adopt the new mask rules.

People are considered fully vaccinated two weeks after their second dose of Pfizer or Moderna's vaccine, or two weeks after the single-dose Johnson & Johnson shot. Currently, the CDC says that more than 163 million Americans have been fully vaccinated, while more than 188 million Americans have received at least one dose of a COVID vaccine. 

This story originally appeared on People.com.