Vaccinated People May Spread the Virus, Though Rarely, C.D.C. Reports

In yet another unexpected and unwelcome twist in the 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.