BOSTON (WHDH) - The Centers for Disease Control and Prevention on Tuesday recommended that vaccinated people return to wearing masks indoors in parts of the United States where the COVID-19 Delta variant is fueling a surge in new cases.

The guidance on masks in indoor public places applies in areas with at least 50 new COVID-19 cases per 100,000 people in the last week, according to the CDC. That includes 60 percent of counties across the county, five of which are in Massachusetts.

The latest data from the CDC shows that Barnstable County has a high COVID transmission rate, with more than 100 new cases per 100,000 residents reported between July 19 and July 25.

Bristol, Suffolk, Dukes, and Nantucket counties all have substantial transmission rates, with between 50 and 99.99 new cases reported in the last week.

With the exception of Franklin County, every other county in the Bay State currently has a moderate transmission rate.

The change in mask guidance comes amid concerns about the variant’s ability to spread among vaccinated people.

There have been more than 750 new virus cases reported in Provincetown since the Fourth of July. Officials say the variant was detected among some cases that have been part of the growing outbreak.

The CDC also recommended indoor masks for all teachers, staff, students and visitors at schools nationwide, regardless of vaccination status.

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