A “substantial” transmission of COVID-19 has reached Delaware County and other Philadelphia region counties, bringing them into the CDC current mask recommendation for the fully vaccinated.

The guidance recommends mask-wearing in public indoor spaces for those places with substantial or high transmission rates, writes Maria Pulcinella for WHYY.org.

The CDC also recommends masks in the schools for teachers, staff and students.

The recommendations came after evidence showed that levels of the virus in the nose of a fully vaccinated person with a breakthrough infection is just as high as someone not vaccinated.

“That’s a big deal,” said NPR’s Maria Godoy on Weekend Edition, “and it’s why the CDC is now advising the vaccinated to mask up indoors in public places.”

The CDC emphasizes that COVID-19 vaccines are still highly effective to prevent infections, as well as severe levels of the disease, and death, and that breakthrough infections for the vaccinated are rare.

As of Aug. 2, 54.1 percent of Delaware County residents have received at least one dosage of vaccine. The fully vaccinated make up 48.7 percent of the county’s population, according to covidactnow.org.

Masks are not currently mandated by Pennsylvania.

Read more at whyy.org about current CDC mask guidelines in Delaware County.