About 74% of 469 COVID-19 cases associated with large gatherings held in Barnstable County, Mass., from July 3 to 17 were among fully vaccinated people, according to data released Friday from the Centers for Disease Control and Prevention.
Why it matters: The data bolsters emerging evidence that vaccinated people have high viral loads and may transmit the Delta variant as easily as those who are unvaccinated.
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Be smart: Despite mild breakthrough cases appearing more common than previously thought, "vaccination is the most important strategy to prevent severe illness and death," the report emphasizes.
The big picture: Presentation slides from the agency leaked Thursday night showed that unpublished research indicated that the Delta variant causes more severe illness in unvaccinated people and spreads as easily as chickenpox.
On Tuesday, the CDC recommended people should wear masks in indoor public settings in areas where COVID-19 transmission is high or substantial regardless of vaccination status.
What they found: Among the vaccinated with a breakthrough infection in Barnstable County, nearly 80% reported mild signs or symptoms, with the most common being cough, headache, sore throat, myalgia and fever.
The people reported being in densely packed indoor and outdoor events at venues that included bars, restaurants, guest houses and rental homes.
Five people were hospitalized, including one patient, who was middle-aged, unvaccinated, and had multiple underlying medical conditions. The remaining four adults were fully vaccinated, varied in age and, two had underlying medical conditions.
As of July 27, no deaths were reported.
A majority of the cases occurred in men with a median age of 40, but is likely due to the health department's sampling from the public events, which were marketed to adult male participants.
What they're saying: "This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation," CDC director Rochelle Walensky said in a statement Friday.
"The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones," she added.
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