Findings from the investigation into the Provincetown COVID-19 outbreak helped formulate the CDC's decision to recommend that all Americans, even if they are vaccinated, wear masks indoors, according to an internal report by the agency.
An internal CDC document obtained by NBC News cites preliminary data suggesting that the delta variant is as contagious as the chickenpox, and even able to spread among vaccinated people.
The report cited the COVID-19 Provincetown outbreak during the July 4 weekend, where more than 900 people were infected with COVID-19 as of Friday, some with the delta variant. Among those living in Massachusetts, 74% of them were fully vaccinated, yet officials said the majority reported mild symptoms of the disease. Seven people were hospitalized.
Get top local stories in Southern California delivered to you every morning. Sign up for NBC LA's News Headlines newsletter.
"There were a number of factors that seem to have created a bit of a perfect storm over the Fourth of July weekend," said Travis Dagenais, who contracted COVID-19 in Provincetown. "Crowding into these old-school New England spaces with low ceilings and little ventilation — if any — it kind of created what feels like a bit of a Petri dish for the Delta variant to really have its way with us."
An investigation by the state's public health department, which was cited in the CDC report, found that those who were vaccinated were transmitting the virus to other vaccinated people.
Many experts say the goal should be shifting the focus from mild case counts to preventing serious disease or death.
"If they’re lucky -- and most people are -- they have what looks like a common cold or allergies or sometimes no symptoms at all," said Dr. Shira Doron of Tufts Medical Center.
Even with breakthrough cases, vaccines are still doing their job by preventing serious illness, said Doron.
"It sometimes allows COVID to cause some symptoms in the upper respiratory tract," said Doron. "It rarely allows that COVID to get down into your lower respiratory tract, into your lungs and other organs."
Gov. Charlie Baker was asked Friday about the Provincetown COVID cluster when speaking to reporters about new vaccine guidance just issued by his administration.
He noted that, in the scheme of things, the outbreak is small, and that it's hard to draw conclusions from an medical event that originated over a rainy July 4 weekend when many people stayed inside.
He also urged residents of Massachusetts who haven't been vaccinated to do so: "You're far less likely to test positive from COVID, to get very sick from COVID, to die from COVID if you're vaccinated."
More on COVID-19
