The hotly anticipated study helped convince the agency to revise its guidance on mask-wearing earlier this week.
The Centers for Disease Control and Prevention has collected data that suggest fully vaccinated Americans who contract the Delta variant carry similar levels of the virus as unvaccinated people who contract the variant.
The finding suggests that people who have gotten Covid-19 shots and then become ill with Delta can transmit the virus to others more readily than seen with other versions of the coronavirus.
The hotly anticipated research helped convince the agency to revise its guidance on mask-wearing earlier this week, when it said vaccinated people should wear masks indoors in areas with high levels of Covid-19 transmission. But CDC had not made the data underlying its decision public until now.
The agency worked with the Massachusetts health department to track a cluster of coronavirus cases in Barnstable County, on Cape Cod, that emerged in the days following July Fourth holiday.
Seventy-four percent of the 469 cases that CDC investigated occurred in fully vaccinated individuals. Genomic sequencing of samples from 119 of 133 individuals in the study showed they had contracted the Delta variant. The vast majority of the vaccinated people in the cluster — about 79 percent — developed clear Covid-19 symptoms, CDC said.
Five people in that group were hospitalized, four of whom were vaccinated. But none died, the study found.
CDC also examined the viral load in 127 of the sick vaccinated people and found it was similar to the level seen in 84 unvaccinated people who contracted the variant. That suggests the vaccinated group was just as able to spread the virus as the unvaccinated group — a finding that prompted CDC to once again recommend that people in areas of high transmissions wear masks indoors regardless of vaccination status.
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