C.D.C. Internal Report Calls Delta Variant as Contagious as Chickenpox

Infections in vaccinated Americans also may be as transmissible as those in unvaccinated people, the document said, and lead more often to severe illness.

The Delta variant is much more contagious, more likely to break through protections afforded by the vaccines and may cause more severe disease than all other known versions of the virus, according to an internal presentation circulated within the Centers for Disease Control and Prevention. Dr. Rochelle P. Walensky, the director of the agency, acknowledged on Tuesday that vaccinated people with so-called breakthrough infections of the Delta variant carry just as much virus in the nose and throat as unvaccinated people, and may spread it just as readily, if less often. But the internal document lays out a broader and even grimmer view of the variant.