The data, detailed in the CDC’s Morbidity and Mortality Weekly Report, provided key evidence that convinced agency scientists to reverse recommendations on mask-wearing and advise that vaccinated individuals wear masks in indoor public settings in some circumstances.
Critically, the study found that vaccinated individuals carried as much virus in their noses as unvaccinated individuals, and that vaccinated people could spread the virus to each other. The CDC was criticized this week for changing its mask guidance without citing unpublished data. The report released Friday contains that data.
“This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation,” CDC director Rochelle Walensky said in a statement. “The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.”
Scientists said the Provincetown outbreak and other recent data on breakthrough infections make clear that the vaccines do work, as hoped, against severe illness and death, but do not offer blanket protection against any chance of infection. Only a handful of people in the outbreak were hospitalized, but four of them were fully vaccinated.
A CDC internal document obtained by The Washington Post estimated that 35,000 vaccinated people a week in the United States are having symptomatic breakthrough infections out of a vaccinated population of more than 162 million. Vaccination coverage is higher than average in Massachusetts, with nearly 70 percent of residents fully vaccinated.
“This shows the delta is formidable,” said Larry Corey, a virologist at the Fred Hutchinson Cancer Research Center. “We can’t take one report of packed bars and extrapolate and say the sky is falling. The sky is not falling. But it does say the vaccine is not infallible.
“Common sense has to be used,” Corey said. “It’s a learning moment, it’s a teaching moment. You can’t overlook the vast data we have on the effectiveness of the vaccine.”
The study’s authors note that Massachusetts has a high vaccination rate and the virus was still able to spread.
“Findings from this investigation suggest that even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status,” they write.
Jennifer Nuzzo, an epidemiologist with the Johns Hopkins Bloomberg School of Public Health, noted that the new CDC guidance on indoor masking for vaccinated people applies to communities with substantial transmission, and Provincetown on July 3 had low levels of virus.
“What this tells us is we need much more context and better data to guide whether and when vaccinated people should wear masks because following CDC’s new guidance wouldn’t have stopped this outbreak from occurring,” Nuzzo said.
The internal CDC document obtained by The Post and published Thursday states that the delta variant is as transmissible as chickenpox and likely to cause more severe infections. That document also shows the CDC believes it needs to revamp its public communications strategy to stress the importance of vaccinations as the best way to crush the pandemic while acknowledging that breakthrough infections are more common than top health officials have previously indicated.
Provincetown is famous for its party scene and is at the tip of Cape Cod in a festive July 4 environment that could have, and did, prove ideal for explosive spread of the more contagious delta variant.
The outbreak has all the hallmarks of a superspreader event, with infected people reporting to public health officials that they gathered in “densely packed indoor and outdoor events that included bars, restaurants, guest houses and rental homes.” The full outbreak, which began July 4, is close to 900 cases, but the analysis included only a subset of them.
About three-quarters of cases occurred in people who were fully vaccinated, and that group had received vaccines from Pfizer-BioNTech, Moderna and Johnson & Johnson.
Scientists at the Broad Institute of MIT and Harvard, a research institute in Cambridge, Mass., involved in the genetic analysis of the outbreak highlighted that this was not a single event. At least five events sparked the outbreak, so it is not possible to blame it on one party or one bar.
“There’s no one person or spot to blame here,” said Daniel Park, group leader for viral computational genomics at the Broad Institute. “The thing that’s catching the attention in national public health is that you can have these types of events; there’s no one particular bar that did any worse than another. Simply mixing that many people in one place with delta going around, with a decently high vaccination rate isn’t quite enough.”
The scientists, along with officials at the Massachusetts Department of Public Health, reported that 79 percent of vaccinated breakthrough infections were symptomatic. Four of five people who were hospitalized were fully vaccinated. They are analyzing the genetic fingerprints of the virus samples taken now to trace chains of transmission and determine how commonly fully vaccinated people were infecting one another.
“We are at an inflection point, I’d say we are in a moment right now a crossroads, a fork in the road where we can either try and take a road to end the pandemic or take a path that will prolong it — a potential nightmare scenario,” said Pardis Sabeti, a geneticist at the Broad and Harvard University.
The study makes clear that vaccines offer significant protection, but do not prevent infection entirely even among the fully vaccinated. On July 3, the Massachusetts Department of Public Health reported a 14-day average of zero covid-19 cases per 100,000 in Barnstable County — but by July 17, that number had increased to 177 cases per 100,000.
“This report demonstrates that vaccination against SARS-CoV-2 is not perfect, particularly in a setting with a highly contagious variant, in a large group in close contact, even if most are vaccinated against the virus,” said Gregg Gonsalves, associate professor at the Yale School of Public Health. “The good news here: If you’re vaccinated, refrain from large group gatherings and mask up, chances are good you’ll be okay. This is not 2020. But we’re not out of the woods.”
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