Unpublished research indicates that the Delta variant causes more severe illness and spreads as easily as chickenpox, and that vaccinated people may transmit the strain as easily as those who are unvaccinated, according to an internal CDC presentation obtained by WashPost.

Why it matters: The data played a key role in the CDC's decision to tell vaccinated people to resume masking indoors, with the presentation calling on health officials to "acknowledge the war has changed," The Post reports.

Zoom in: The document affirmsthat the vaccines provide substantial protection against serious illness and death, but warns that mild breakthrough cases may be more common than previously thought.

• It stresses the daunting "communication challenges" that this new phase could bring, as the agency looks to convince a skeptical public of the effectiveness of the vaccines while moving the "goal posts of success in full public view," The Post writes.
• CDC Director Rochelle Walensky briefed members of Congress on much of the data Thursday, and the agency is expected to release it in full on Friday.

Read the presentation.