Behind the Masks, a Mystery: How Often Do the Vaccinated Spread the Virus?

The recommendation that vaccinated people in some parts of the country dust off their masks was based largely on one troublesome finding, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.

New research showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in the nose and throat, she said in an email responding to questions from The New York Times.

The finding contradicts what scientists had observed in vaccinated people infected with previous versions of the virus, who mostly seemed incapable of infecting others.

That conclusion dealt Americans a heavy blow: People with so-called breakthrough infections — cases that occur despite full vaccination — of the Delta variant may be just as contagious as unvaccinated people, even if they have no symptoms.

That means fully immunized people with young children, aging parents, or friends and family with weak immune systems will need to renew vigilance, particularly in high-transmission communities. Vaccinated Americans may need to wear masks not just to protect themselves, but everyone in their orbit.

There are 67,000 new cases per day on average in the United States, as of Thursday. If vaccinated people are transmitting the Delta variant, they may be contributing to the increases — although probably to a far lesser degree than the unvaccinated.

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.

It’s still unclear how common breakthrough infections are and how long the virus persists in the body in those cases, Dr. Walensky said. Breakthroughs are rare, and unvaccinated people account for the bulk of virus transmission, she said.

Regardless, the data that the C.D.C. is reviewing suggest that even fully immunized people can be unwilling vectors for the virus. “We believe at individual level they might, which is why we updated our recommendation,” Dr. Walensky said in her email to The Times.

The conclusion also suggests that vaccinated people who are exposed to the virus should get tested, even if they feel fine. (In Britain, vaccinated people who are contacts of a known case are