The delta variant of the coronavirus appears to cause more severe illness than earlier variants and spreads as easily as chickenpox, according to an internal federal health document that argues officials must “acknowledge the war has changed.”.

It captures the struggle of the nation’s top public health agency to persuade the public to embrace vaccination and prevention measures, including mask-wearing, as cases surge across the United States and new research suggests vaccinated people can spread the virus.

The document strikes an urgent note, revealing the agency knows it must revamp its public messaging to emphasize vaccination as the best defense against a variant so contagious that it acts almost like a different novel virus, leaping from target to target more swiftly than Ebola or the common cold.

It cites a combination of recently obtained, still-unpublished data from outbreak investigations and outside studies showing that vaccinated individuals infected with delta may be able to transmit the virus as easily as those who are unvaccinated.

CDC scientists were so alarmed by the new research that the agency earlier this week significantly changed guidance for vaccinated people even before making new data public.

The data and studies cited in the document played a key role in revamped recommendations that call for everyone — vaccinated or not — to wear masks indoors in public settings in certain circumstances, a federal health official said.

The document outlines “communication challenges” fueled by cases in vaccinated people, including concerns from local health departments about whether coronavirus vaccines remain effective and a “public convinced vaccines no longer work/booster doses needed.”.

“Although it’s rare, we believe that at an individual level, vaccinated people may spread the virus, which is why we updated our recommendation,” according to the federal health official, who spoke on the condition of anonymity because they were not authorized to speak publicly.

The presentation came two days after Walensky announced the reversal in guidance on masking among people who are vaccinated.

The document presents new science but also suggests a new strategy is needed on communication, noting that public trust in vaccines may be undermined when people experience or hear about breakthrough cases, especially after public health officials have described them as rare.

The document includes CDC data from studies showing that the vaccines are not as effective in immunocompromised patients and nursing home residents, raising the possibility that some at-risk individuals will need an additional vaccine dose.

The internal document contains some of the scientific information that influenced the CDC to change its mask guidance.

Orenstein, associate director of the Emory Vaccine Center, said he was struck by data showing that vaccinated people who became infected with delta shed just as much virus as those who were not vaccinated.

Kathleen Neuzil, a vaccine expert at the University of Maryland School of Medicine, said getting more people vaccinated remains the priority, but the public may also have to change its relationship to a virus almost certain to be with humanity for the foreseeable future