A cluster of cases in Provincetown is proving a model for breakthrough infections, prompting public health leaders to rethink mask rules for the public. On Friday, Massachusetts' own Department of Public Health issued new masking guidance, recommending vaccinated adults wear masks indoors in some cases.

Dr. Katherine Gergen Barnett of Boston Medical Center joined Boston Public Radio on Friday to go over the latest developments.

The new DPH guidance recommends fully vaccinated people wear masks indoors if they have a weakened immune system or are at increased risk for severe disease, or if someone in their household has a weakened immune system, is at increased risk for severe disease, or is unvaccinated — like young children.

"I'm thrilled they came out with that at least," said Gergen Barnett, who noted she would go further and reinstate mask mandates.

"I think the more we split hairs with our recommendations ... I think the more confusion that happens," she said.

Gergen Barnett said as leaders think about how to safely return to school in fall, she "would highly recommend we create mask mandates again indoors."

"I think it's just the safer thing to do," she said.

Just two months after the CDC relaxed mask rules for the fully vaccinated population, as the Delta variant becomes more prominent — and is responsible for cases even among vaccinated people — the CDC is now recommending that people in high transmission areas to wear masks inside.

An internal memo published by the Washington Post revealed that the CDC is worried "the war is changing."

"While I don't love war terminology all the time, I think it's probably very accurate," Gergen Barnett said.

"The important thing is that if you're vaccinated you are still three times less likely to get COVID-19, and are 10 times less likely to die," said Gergen Barnett. "But, if you do get COVID-19 if you're vaccinated, then you have just as much viral load as somebody who has not been vaccinated. So that is really new data, that is very specific to Delta, that is extremely concerning."

Gergen Barnett said that data is "very compelling for why masks are necessary."

Gergen Barnett also discussed the likelihood that booster shots might be announced in the near future, vaccine mandates, and took calls from listeners.

Dr. Katherine Gergen Barnett is the Vice Chair of Primary Care Innovation and Transformation and the Program Director in the Department of Family Medicine at Boston Medical Center.