The Delta variant is much more contagious, is more likely to breach vaccine protection, and can cause more serious illness than any other known version of the virus, according to an internal presentation spread within the Centers for Disease Control and Prevention.
Dr. Rochelle P. Walensky, the agency’s director, admitted on Tuesday that vaccinated people with so-called breakthrough infections of the Delta variant carry just as much virus in their nose and throat as unvaccinated people and can spread it just as easily, albeit less often.
But the internal document sets out a broader and even more somber view of the variant.
The delta variant is more transmissible as the viruses that cause MERS, SARS, Ebola, the common cold, seasonal flu, and smallpox, and according to the document copied by the New York Times, it is as contagious as chickenpox.
The immediate next step for the agency is to “realize that the war has changed,” the document reads. The content was first reported by the Washington Post on Thursday evening.
The tone of the document echoes CDC scientists’ concern about the spread of Delta across the country, said a federal official who saw the research described in the document. The agency plans to publish further data on the variant on Friday.
“The CDC is very concerned about the incoming data that Delta is a very serious threat that requires action now,” the official said.
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In the US, there were an average of 71,000 new cases a day as of Thursday. The new data suggests that vaccinated people spread the virus and contribute to these numbers – albeit likely to a far lesser extent than those who were not vaccinated.
Dr. Walensky has called transmission by vaccinated people a rare occurrence, but other scientists have suggested it is more common than previously thought.
The agency’s new masking guidelines for vaccinated individuals, introduced on Tuesday, were based on information contained in the document. The CDC recommended that vaccinated people wear masks indoors in public settings in communities with high virus transmission levels.
Updated
July 31, 2021 at 11:50 p.m. ET
However, the internal document indicates that even this recommendation may not go far enough. “In view of the higher transferability and current vaccination protection, universal masking is essential,” the document says.
The agency’s data suggests that people with weak immune systems should wear masks even in places with low virus transmission. This should include vaccinated Americans who are in contact with young children, older adults, or other vulnerable people.
According to the July 24 CDC quoted in the internal presentation, there are about 35,000 symptomatic infections per week among 162 million Americans vaccinated. However, the agency does not track all mild or asymptomatic infections, so the actual incidence may be higher.
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Infection with the delta variant produces amounts of virus in the airways that are ten times higher than in people infected with the also highly contagious alpha variant, the document says.
According to a recent study, the amount of virus in a person infected with Delta is a thousand times higher than in people infected with the original version of the virus.
The CDC document draws on data from several studies, including an analysis of a recent Provincetown, Massachusetts outbreak that began in the city after the July 4th celebrations. By Thursday, that cluster had grown to 882 cases. About 74 percent had been vaccinated, said the local health authorities.
A detailed analysis of the prevalence of the cases showed that people infected with Delta carry enormous amounts of virus in their nose and throat regardless of vaccination status, according to the CDC document.
“This is one of the most impressive examples of citizen science I’ve seen,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “The people involved in the Provincetown outbreak meticulously created lists of their contacts and exposures.”
Infection with the Delta variant can be more likely to lead to serious illness, the document says. Studies from Canada and Scotland found that people infected with the variant were more likely to be hospitalized, while research in Singapore suggested that they were more likely to need oxygen.
Still, the CDC’s numbers show the vaccines are highly effective at preventing serious illness, hospitalization and death in people who have been vaccinated, experts said.
“Overall, Delta is the disturbing variant that we already knew it was,” said John Moore, a virologist at Weill Cornell Medicine in New York. “But the sky is not falling and vaccinations are still very protective against the worst of the consequences.”
