The Provincetown data contained two startling details: nearly three-quarters of infected people were fully vaccinated, the CDC said, and samples showed that the amount of virus infected people carried—or viral load—was similar between vaccinated and unvaccinated patients. The CDC concluded from the latter that vaccinated people who become infected might spread the Delta variant as readily as the unvaccinated.
The meaning of some of the viral load data has been disputed. Inside the CDC, some officials disagree with the agency’s conclusion that vaccinated people who become infected may spread the virus as readily as the unvaccinated, and argue that more testing needs to be done, including tests that measure how infectious virus particles are, according to a person familiar with the matter.
“There’s no one-to-one relationship between high viral load and infectivity, but we’re always making decisions based on imperfect data,” said Tom Frieden, who headed the CDC from 2009 to 2017.
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