BOSTON (SHNS/WWLP) – In parts of the country where the Delta variant is fueling a surge of COVID-19 cases, including in parts of Massachusetts, even people who are fully vaccinated against the virus should go back to wearing a mask when in public indoor spaces, the U.S. Centers for Disease Control said Tuesday.

The CDC is now recommending that everyone “wear a mask indoors in public if you are in an area of substantial or high transmission,” a designation that included 63.45 percent of U.S. counties as of Tuesday afternoon. In Massachusetts, Barnstable County is the only area of high transmission, and Bristol, Dukes, Nantucket, and Suffolk counties are considered areas of substantial transmission.

“In recent days, I have seen new scientific data from recent outbreak investigations showing that the Delta variant behaves uniquely differently from past strains of the virus that cause COVID-19,” CDC Director Dr. Rochelle Walensky said during a Tuesday media briefing.

Fully vaccinated people including children should begin wearing masks indoors again in places with high COVID-19 transmission rates. Teachers, school staff, and children are also recommended to wear masks again for the upcoming school year.

“Going back to the basic prevention method that we have been using the past year, which is the mask, I think it’s reasonable for people to consider,” said Dr. Mohammad E. Abu at Baystate Health.

Statewide, most counties are at moderate levels of transmission. In the eastern part of the state, however, Barnstable County is in the red…with high transmission, COVID-19 clusters are being detected in Provincetown.

“This new science is worrisome and unfortunately warrants an update to our recommendations.” She added, “This is not a decision that we or CDC has made lightly.” The updated federal guidance marks a reversal from May when the CDC eased mask-wearing guidance for the fully vaccinated and President Joe Biden declared it a “great day for America.” Since then, the more transmissible Delta variant of COVID-19 has propelled an increase in case counts and hospitalizations in Massachusetts and across the country.

22News spoke with one person who said that they are considering masking up again.

“I plan on wearing a mask probably more during the cold and flu season just so I don’t spread it to other people,” said Andrew Closser of Easthampton.

The CDC said the Delta variant was responsible for about 1.4 percent of all COVID-19 cases in the two-week period that ended May 8. As of the beginning of July, the mutant was responsible for about 63 percent of all new cases. On May 13, when the CDC said that fully vaccinated people could take off their masks, Massachusetts was averaging about 546 new cases of COVID-19 each day but the number was falling rapidly. By the end of June, the state was averaging just 64 daily new cases.

On Tuesday, the Mass. Department of Public Health reported 657 new cases and said the average now stands at about 400 daily new cases. The CDC on Tuesday also recommended indoor mask-wearing for teachers, staff, students, and visitors at schools, regardless of vaccination status. In Massachusetts, Gov. Charlie Baker has said he has no “plans to change our current policies with respect to school in the fall” but on Monday said he was waiting to hear from the CDC and other governors on the issue.

“I’m going to continue to collect data from the CDC and from other points of view on this that I think are particularly pertinent to this,” he said. “But most of the governors that I’ve talked to have said that they’re waiting to hear more from the feds before they make their own decisions on this.”