In yet another unexpected and unwelcome twist in the nation’s pandemic, fully immunized people with so-called breakthrough infections of the
Delta variant may spread the virus to others just as easily as unvaccinated people, the Centers for Disease Control and Prevention said in a report published on Friday.
The vaccines remain powerfully effective against severe illness and death, and infections in vaccinated people are thought to be comparatively rare. But the revelation follows a series of other findings this week about the Delta variant, all of which have upended scientists’ understanding of the coronavirus.
In the report published on Friday, the agency described a single outbreak in Provincetown, Mass., that quickly mushroomed to nearly 469 cases in the state as of Thursday, three-quarters of whom were fully immunized.
An internal agency document, which was obtained on Thursday night by The New York Times, raised even more harrowing questions about the virus and its trajectory. Taken together, the data gathered by the C.D.C. throw into question the country’s plans to return to offices and schools this fall, and revive difficult questions about masking, testing and other precautions that Americans had hoped were behind them.
Most immediately, the research informed the agency’s decision this week to advise even vaccinated Americans to resume wearing masks in indoor public areas in communities where the virus is surging.
Even the vaccinated carry high virus levels if they become infected, the agency concluded, making it likely they can transmit the virus as often as the unvaccinated. If so, they may be contributing to increases in new infections — although probably to a far lesser degree than the unvaccinated.
“We spent so much time and energy and treasure trying to figure out this damn virus last year, and how it works and all the things it does,” said Dr. Robert Wachter, chairman of the Department of Medicine at the University of California, San Francisco.
Learning just how different the Delta variant is from the original virus is “just jarring,” he added. “The brain doesn’t like to keep being jerked around like this.”
Studies of outbreaks have shown that Delta is much more contagious than the original virus or the seasonal flu and as contagious as chickenpox, according to the internal document circulated within the C.D.C.
