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The document’s tone reflects alarm among the centre’s scientists about Delta’s spread across the country,said a federal official who has seen the research described in the document. The agency is expected to publish additional data on the variant.

“The CDC is very concerned with the data coming in that Delta is a very serious threat that requires action now,” the official said.

There were 71,000 new cases per day on average in the United States,as of Thursday. The new data suggest that vaccinated people are spreading the virus and contributing to those numbers — although probably to a far lesser degree than the unvaccinated.

Walensky has called transmission by vaccinated people a rare event,but other scientists have suggested it may be more common than once thought.

The agency’s new masking guidelines for vaccinated people,introduced on Tuesday,were based on the information presented in the document. The CDC now recommends that vaccinated people wear masks indoors in public settings in communities with high transmission of the virus.

But the internal document hints that even that recommendation may not go far enough. “Given higher transmissibility and current vaccine coverage,universal masking is essential,” the document said.

The agency’s data suggest that people with weak immune systems should wear masks even in places that do not have high transmission of the virus. So should vaccinated Americans who are in contact with young children,older adults,or otherwise vulnerable people.

There are roughly 35,000 symptomatic infections per week among 162 million vaccinated Americans,according to data collected by the CDC as of July 24 that was cited in the internal presentation. But the agency does not track all mild or asymptomatic infections,so the actual incidence may be higher.

Infection with the Delta variant produces virus amounts in the airways that are tenfold higher than what is seen in people infected with the alpha variant,which is also highly contagious,the document noted.

The amount of virus in a person infected with Delta is a thousandfold more than what is seen in people infected with the original version of the virus,according to one recent study.

The agency’s document relies on data from multiple studies,including an analysis of a recent outbreak in Provincetown,Massachusetts,which began after the town’s Fourth of July festivities. By Thursday,that cluster had grown to 882 cases. About 74 per cent were vaccinated,local health officials have said.

Detailed analysis of the spread of 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 have seen,” said Dr Celine Gounder,an infectious disease specialist at Bellevue Hospital Centre in New York. “The people involved in the Provincetown outbreak were meticulous in making lists of their contacts and exposures.”

Infection with the Delta variant may be more likely to lead to severe illness,the document noted. Studies from Canada and Scotland found that people infected with the variant are more likely to be hospitalised,while research in Singapore indicated that they are more likely to require oxygen.

Still,the CDC’s figures show that the vaccines are highly effective in preventing serious illness,hospitalisation and death in vaccinated people,experts said.

“Overall,Delta is the troubling variant we already knew it was,” said John Moore,a virologist at Weill Cornell Medicine in New York. “But the sky isn’t falling and vaccination still protects strongly against the worse outcomes.”

The New York Times