“They’re making a claim that people with delta who are vaccinated and unvaccinated have similar levels of viral load, but nobody knows what that means,” said Gregg Gonsalves, an associate professor at the Yale School of Public Health. “It’s meaningless unless we see the data.”

When CDC Director Rochelle Walensky spoke to reporters Tuesday, she cited the “new scientific data” but provided limited details about how the research was done. She said the data comes from outbreak investigations in which researchers compared delta infections among vaccinated and unvaccinated people.

The data will be “published imminently,” according to a federal official knowledgeable about the research but who was not authorized to be a spokesperson for the government.

“These data were alarming and recently presented,” the official said Wednesday. “We saw the data and thought it was urgent enough to act — in the context of a steeply rising, preventable fourth surge of covid-19.”