Delta variant appears to spread as easily as chickenpox

Washington (CNN)The Delta coronavirus variant surging across the United States appears to cause more severe illness and spread as easily as chickenpox, according to an internal document from the US Centers for Disease Control and Prevention.

The document -- a slide presentation -- outlines unpublished data that shows fully vaccinated people might spread the Delta variant at the same rate as unvaccinated people.

The CDC is scheduled to publish data Friday that will back Walensky's controversial decision to change guidance for fully vaccinated people.

She said the CDC was recommending that even fully vaccinated people wear masks indoors in places where transmission of the virus is sustained or high.

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.

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.

But vaccinated people are safer, the document indicates.

"Vaccines prevent more than 90% of severe disease, but maybe 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 against surging across the US -- especially in areas where fewer people are vaccinated.

The US averaged more than 61,300 new daily cases over the last week -- an average that's generally risen since the country hit a 2021 low of 11,299 daily cases on June 22, according to Johns Hopkins University data.

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.