The C.D.C. has not yet published its data, frustrating experts who want to understand the basis for the change of heart on masks. But four scientists familiar with the research said it was compelling and justified the C.D.C.’s advice that the vaccinated wear masks again in public indoor spaces.

The study was conducted by a group outside the C.D.C., the scientists said, and the agency is working quickly to analyze and publish the results…

The coronavirus vaccines are injected into muscle, and the antibodies produced in response mostly remain in the blood. Some antibodies may make their way to the nose, the main port of entry for the virus, but not enough to block it.

“The vaccines — they’re beautiful, they work, they’re amazing,” said Frances Lund, a viral immunologist at the University of Alabama at Birmingham. “But they’re not going to give you that local immunity.”