The Centers for Disease Control and Prevention has issued a stern warning about the delta variant of the coronavirus: “Acknowledge the war has changed.” Now, even vaccinated people are able to readily spread the virus.

That is part of the message from a recent internal presentation prepared by the CDC detailing the dangers posed by the delta variant, which has already led to a spike in cases in the U.S. The document, obtained Friday by NBC News and first published by The Washington Post, explains the scientific background behind the agency’s change in mask guidance earlier this week.

It concludes that the delta variant of the coronavirus is “highly contagious, likely to be more severe” and that “breakthrough infections may be as transmissible as unvaccinated cases.”

Infections with the delta variant lead to higher levels of virus in the body, even in breakthrough cases in fully vaccinated individuals, the document said. Virus levels can be as high in breakthrough cases as in unvaccinated people, even if vaccinated people don’t get nearly as sick. What’s more, these higher levels also persist for longer than was seen with previous strains, meaning an infected person is likely contagious for longer.

Still, the vast majority of transmission is occurring among unvaccinated individuals, according to the CDC.

Vaccines continue to be effective, particularly at preventing severe disease, according to the document. But they may not be as good at preventing infection or transmission of the delta variant.

That’s a change from previous variants. The vaccines were very effective at preventing transmission of the alpha variant, which was the dominant strain in the country earlier this year when the CDC first said that vaccinated people do not need to wear masks.

“Therefore, more breakthrough and more community spread despite vaccination,” the document states.

Dr. Ashish Jha, dean of the Brown University School of Public Health, tweeted Thursday night that the CDC presentation was “insightful & largely reassuring,” emphasizing that the data shows the delta variant is highly contagious but the vaccines continue to prevent most infections and almost all hospitalizations.

Delta variant is as contagious as chickenpox, CDC document says

The document notes that the risk of infection is threefold lower in vaccinated people, and the risk of severe disease or death is at least tenfold lower in vaccinated people.

The document also provided more concrete numbers on breakthrough infections, estimating that at current levels, there are 35,000 symptomatic breakthrough infections per week among the 162 million fully vaccinated adults in the U.S. The agency stopped providing public information on most breakthrough infections in April, when the tally hit 10,000. From that point on, the CDC website only posted data on breakthrough infections that led to hospitalization or death.

It also details just how much more contagious the delta variant is than earlier versions of the coronavirus. A chart included in the document states that it is more transmissible than the flu, the common cold and even smallpox and is on par with chickenpox, considered among the most contagious common viruses.

The document comes as the U.S. is grappling with a surge in coronavirus cases, particularly in areas where vaccination rates remain low. Parts of Missouri and Louisiana have reported rising hospitalizations on par with the worst days of the pandemic. Daily deaths in the U.S. have steadily mounted in recent weeks but remain significantly lower than in the spring.