The document - a slide presentation - outlines data that shows fully vaccinated people might spread the delta variant at the same rate as unvaccinated people.
CDC shares 'pivotal discovery' on COVID-19 breakthrough infections that led to new mask guidance
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CDC Director Dr. Rochelle Walensky confirmed the authenticity of the document, which was first reported by The Washington Post.
"I think people need to understand that we're not crying wolf here. This is serious," she told CNN.
"It's one of the most transmissible viruses we know about. Measles, chickenpox, this -- they're all up there."
The CDC published the data Friday that backed Walensky's controversial decision to change guidance for fully vaccinated people. She said Tuesday the CDC was recommending that even fully vaccinated people wear masks indoors in places where transmission of the virus is sustained or high.
And she said everyone in schools -- students, staff and visitors -- should wear masks at all times.
"The measures we need to get this under control -- they're extreme. The measures you need are extreme," Walensky told CNN.
She said the data in the report did not surprise her. "It was the synthesis of the data all in one place that was sobering," she said.
The CDC presentation says the delta variant is about as transmissible as chickenpox, with each infected person, on average, infecting eight or nine others. The original lineage was about as transmissible as the common cold, with each infected person passing the virus to about two other people on average.
That infectivity is known as R0.
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"When you think about diseases that have an R0 of eight or nine -- there aren't that many," Walensky told CNN.
And if vaccinated people get infected anyway, they have as much virus in their bodies as unvaccinated people. That means they're as likely to infect someone else as unvaccinated people who get infected.
"The bottom line was that, in contrast to the other variants, vaccinated people, even if they didn't get sick, got infected and shed virus at similar levels as unvaccinated people who got infected," Dr. Walter Orenstein, who heads the Emory Vaccine Center and who viewed the documents, told CNN.
But vaccinated people are safer, the document indicates.
"Vaccines prevent more than 90% of severe disease, but may be less effective at preventing infection or transmission," it reads. "Therefore, more breakthrough and more community spread despite vaccination."
It says vaccines reduce the risk of severe disease or death 10-fold and reduce the risk of infection three-fold.
The presentation also cites three reports that indicate the delta variant -- originally known as B.1.617.2 -- might cause more severe disease.
The CDC, the document advises, should "acknowledge the war has changed." It recommends vaccine mandates and universal mask requirements.
The virus is once again surging across the United States - especially in areas where fewer people are vaccinated.
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The country averaged more than 61,300 new daily cases over the last week -- an average that's generally risen since a 2021 low of 11,299 daily cases on June 22, according to Johns Hopkins University data.
"The number of cases we have now is higher than any number we had on any given day last summer," Walensky told CNN.
As of Wednesday, cases have risen in all but one state in the past seven days compared with the week before, according to Johns Hopkins.
"The one thing I will say is I've been heartened in the past couple of days to see more people taking action in response to the fact that it's bad -- more organizations, businesses, states, localities taking the action that's needed to get us out of this," Walensky said.
The CDC document walks through new "communication challenges" as a result of breakthrough infections, along with the need to retool public health messaging to highlight vaccination as the best defense against the delta variant.
The agency should "improve (the) public's understanding of breakthrough infections" and "improve communications around individual risk among vaccinated," it says.
Earlier Thursday, President Joe Biden announced a number of new steps his administration will take to try to get more Americans vaccinated, including requiring that all federal employees must attest to being vaccinated against Covid-19 or face strict protocols.
"This is an American tragedy. People are dying -- and will die -- who don't have to die. If you're out there unvaccinated, you don't have to die," Biden said during remarks at the White House. "Read the news. You'll see stories of unvaccinated patients in hospitals, as they're lying in bed dying from COVID-19, they're asking, 'Doc, can I get the vaccine?' The doctors have to say, 'Sorry, it's too late.' "