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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 e-mail.

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 CDC has not yet published its data, frustrating experts who want to understand the basis for the change of heart on masks.

The research was conducted by people outside the CDC, the scientists said, and the agency is working quickly to analyze and publish the results. The agency expects to publish the research Friday.

It's still unclear how common breakthrough infections are and how long the virus persists in the body in those cases. Breakthroughs are rare, and unvaccinated people account for the bulk of virus transmission, Walensky said. Regardless, the data that the CDC 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," Walensky said.

The conclusion also suggests that vaccinated people who are exposed to the virus should get tested, even if they feel fine.

The new data do not mean that the vaccines are ineffective. The vaccines still powerfully prevent severe illness and death, as they were meant to, and people with breakthrough infections very rarely end up in a hospital.