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, it says 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 findings, some of which are considered preliminary, on the dangers posed by the delta variant, which has already led to a spike in cases in the United States. 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 is “highly contagious, likely to be more severe” and that “breakthrough infections may be as transmissible as unvaccinated cases.”

Researchers have been focusing on viral load — a term for just how much of the virus is present in infected peoples’ bodies — which can affect transmissibility and severity. 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, outbreaks are occurring mostly among unvaccinated individuals, according to CDC Director Dr. Rochelle Walensky.

“I think we still largely are in a pandemic of the unvaccinated,” Walensky said Tuesday in a press briefing to announce updated guidance on wearing masks. “The vast majority of transmission, the vast majority of severe disease, hospitalization and death is almost exclusively happening among unvaccinated people.”

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Susan Butler-Wu, a clinical microbiologist at the University of Southern California, said that the CDC document highlighted reasons for concern but did not indicate that vaccinated people are at greater risk.

“I sort of view this as a hold up, you guys, we might have a problem, put your mask back on,” Butler-Wu said. “But I think the way this is going to get misinterpreted is that vaccines don’t work. And I don’t think that that’s the case at all, based on the data that they’ve presented.”

​​Indeed, 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.

But breakthrough infections are expected, particularly as the virus changes and the pandemic evolves, said Gigi Gronvall, an associate professor at the Johns Hopkins Bloomberg School of Public Health. Gronvall said the CDC document provides more evidence that even as breakthrough infections are occurring, the vaccines remain highly effective.

“The vaccine is not a force field that stops any virus from crossing your path,” she said. “It’s an education program for your immune system so that if you do have an intruder, a virus that infects you, your immune system is ready to pounce.”

The strength of the immune response, however, is variable and largely depends on individual circumstances, including how much virus a person is exposed to and the specific variant involved. Gronvall said the underlying message is the same: unvaccinated people are disproportionately at risk.

“Everybody is paying attention to breakthrough infections but the main risk is with the unvaccinated,” she said. “They are at risk from the virus and they are causing other people to have risks that they shouldn’t have to have.”

Dr. Ashish Jha, dean of the Brown University School of Public Health, similarly 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.

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.

One piece of evidence cited in the document came from an outbreak in Provincetown, Massachusetts, where 74 percent of cases were in fully vaccinated individuals. In a report on the outbreak published Friday by the CDC, researchers said that the delta variant was implicated in 89 percent of cases, and in the breakthrough cases, 79 percent of people developed symptoms.

Of note, PCR tests, which are used to determine if someone is infected, showed similar levels of the virus in vaccinated people compared to unvaccinated people. PCR results, the CDC wrote, “might reflect the level of infectious virus.” In other words, it could suggest that vaccinated people are as contagious as unvaccinated people.

But Butler-Wu noted that the CDC report didn’t say whether virus levels were higher in fully vaccinated people who developed symptoms, compared to fully vaccinated people who were asymptomatic. “That’s what we really need to dig into,” she said, to determine if fully vaccinated people can spread the virus when they don’t have symptoms.

The CDC report only included cases in Massachusetts residents through July 26 — a total of 469 individuals. As of July 29, that number had risen to 531, with an additional 351 cases reported in residents of other states, for a total of 882 cases tied to the outbreak, Provincetown Town Manager Alex Morse said on Facebook. Seven people have been hospitalized, and no deaths have been reported.

The internal CDC 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 increased transmissibility means everyone, regardless of vaccination status, should be cautious in order to stave off new outbreaks, said Dr. Mark Mulligan, director of the NYU Langone Vaccine Center.

“The safe thing to do is to layer on the non-pharmaceutical interventions like masking and social distancing because unfortunately, we’re going backwards,” he said, adding that he hopes the current situation will motivate more people to get vaccinated.

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.

The severity of the current wave of outbreaks across the country adds urgency to vaccination efforts. Promoting vaccine uptake, especially in areas that have lagged behind, will be critical to protect vulnerable communities and prevent other variants from emerging, Butler-Wu said.

“The vaccines are our main way out,” she said. “If you think about sharks circling around and trying to feed — that’s Covid now if you’re unvaccinated.”


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