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July 31, 2021, 9:55 a.m. EDT

‘As transmissible as chickenpox.’ Here’s the CDC report on the delta variant that led to new mask policy.

By Barbara Kollmeyer

The delta variant of COVID-19 is “as transmissible as chickenpox,” and could spread as easily from vaccinated persons as the unvaccinated, according to an internal document from U.S. health officials.

Health officials must “acknowledge the war has changed,” regarding the variant that has spread quickly across the U.S. and rest of the world, said an internal Centers for Disease Control and Prevention (CDC) presentation obtained by The Washington Post, which reported the findings on Thursday.

The data, from recent, yet-to-be published studies, is believed to have driven fresh guidelines Tuesday from the CDC advising even vaccinated persons wear mask indoors in parts of the U.S. with surging cases. The document itself states that the data is preliminary and subject to change.

Read: As COVID-19 cases surge and concerns about breakthrough infections grow, masks come back and anxiety kicks in

CDC Director Rochelle Walensky privately briefed Congress members on Thursday, based on much of what was contained in the slides, according to a source quoted by The Post.

Robert Wachter, chairman of the Department of Medicine at the University of California at San Francisco, said on Twitter that some of the information was already known, but other parts were new and more concerning.

“My overall take: As the document says, “the war has changed.” Data argues that universal masking is critical to block spread of delta – a more infectious, and possibly more serious virus – particularly if it’s true that vaccinated folks can be part of the chain of spread,” Wachter tweeted , adding that a “new and far more aggressive” vaccination strategy is needed.

Read : Pfizer says immunity drops to 83% within four months in people who got its COVID-19 shot, further bolstering the company case for a booster

As Watchter explained, the document shows data estimating an R0 of 5-9 for the delta strain, vs. the 2-3 for the original. The R number rates how COVID-19 or any disease can spread and represents how many people one sick person can pass the illness onto on average.

“We’ve been estimating R0 of ~6 for delta, or ~2x as infectious as original. It may be a bit worse than that,” he tweeted . The delta variant is “more transmissible” than MERS, SARS, Ebola, the common cold, the seasonal flu and smallpox, and caught as easy as chickenpox, the document shows.

He also said the data shows the levels of viral shedding is around 18 days versus 13 for original strains, which could raise new questions over quarantines. And fully vaccinated people infected with the the delta variant carry a huge amount of the virus, and can be just as contagious as unvaccinated people.

What is stressed in the presentation is that faster and more urgent action is needed to get a handle on the spread of the variant, which the documents authors see as more of a threat to vulnerable populations and older people are at higher risk of death and hospitalization even if they are vaccinated.

The document acknowledges that vaccine breakthrough cases “may reduce public confidence in vaccines,” and is causing communication challenges for public health officials. Among those, the “public is convinced vaccines no longer work/booster doses needed,” therefore officials must “update communications describing breakthrough cases as ‘rare’ or as a ‘small percentage’ of cases.”

One slide estimates 35,000 symptomatic infections a week among 162 million vaccinated Americans, though it finds a far greater risk of disease, hospitalization and death among the unvaccinated.

An official told The Post that the data would be published in full on Friday. The CDC did not immediately respond to a request for comment.

Link to MarketWatch's Slice.