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It cites a combination of recently obtained, still-unpublished data from outbreak investigations and outside studies showing that vaccinated individuals infected with delta may be able to transmit the virus as easily as those who are unvaccinated. Vaccinated people infected with delta have measurable viral loads similar to those who are unvaccinated and infected with the variant.
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One of the slides states that there is a higher risk among older age groups for hospitalization and death relative to younger people, regardless of vaccination status. Another estimates that there are 35,000 symptomatic infections per week among 162 million vaccinated Americans.
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“Although it’s rare, we believe that at an individual level, vaccinated people may spread the virus, which is why we updated our recommendation,” according to the federal health official, who spoke on the condition of anonymity because they were not authorized to speak publicly. “Waiting even days to publish the data could result in needless suffering and as public health professionals we cannot accept that.”
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The CDC’s revised mask guidance stops short of what the internal document calls for. “Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant,” it states.

The document makes clear that vaccination provides substantial protection against the virus. But it also states that the CDC must “improve communications around individual risk among [the] vaccinated” because that risk depends on a host of factors, including age and whether someone has a compromised immune system.

The document includes CDC data from studies showing that the vaccines are not as effective in immunocompromised patients and nursing home residents, raising the possibility that some at-risk individuals will need an additional vaccine dose.
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“I think the central issue is that vaccinated people are probably involved to a substantial extent in the transmission of delta,” Jeffrey Shaman, a Columbia University epidemiologist, wrote in an email after reviewing the CDC slides. “In some sense, vaccination is now about personal protection — protecting oneself against severe disease. Herd immunity is not relevant as we are seeing plenty of evidence of repeat and breakthrough infections.”
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Kathleen Neuzil, a vaccine expert at the University of Maryland School of Medicine, said getting more people vaccinated remains the priority, but the public may also have to change its relationship to a virus almost certain to be with humanity for the foreseeable future.

“We really need to shift toward a goal of preventing serious disease and disability and medical consequences, and not worry about every virus detected in somebody’s nose,” Neuzil said. “It’s hard to do, but I think we have to become comfortable with coronavirus not going away.”