The delta variant of the coronavirus appears to cause more severe illness than earlier variants and spreads as easily as chickenpox, according to an internal federal health document that argues officials must “acknowledge the war has changed.”…

Walter A. Orenstein, associate director of the Emory Vaccine Center, said he was struck by data showing that vaccinated people who became infected with delta shed just as much virus as those who were not vaccinated. The slide references an outbreak in Barnstable County, Mass., where vaccinated and unvaccinated people shed nearly identical amounts of virus.

“I think this is very important in changing things,” Orenstein said.

A person working in partnership with the CDC on investigations of the delta variant, who spoke on the condition of anonymity because they were not authorized to speak, said the data came from a July 4 outbreak in Provincetown, Mass. Genetic analysis of the outbreak showed that people who were vaccinated were transmitting the virus to other vaccinated people. The person said the data was “deeply disconcerting” and a “canary in the coal mine” for scientists who had seen the data.