The internal documents also cite studies from Canada, Singapore and Scotland showing that the delta variant may poses a greater risk for hospitalization, intensive care treatment and death than the alpha variant, first detected in the United Kingdom.
Since January, people who got infected after vaccination make up an increasing portion of hospitalizations and in-hospital deaths among COVID-19 patients, according to the documents. That trend coincides with the spread of the delta variant.
But the CDC emphasizes that COVID-19 vaccines are still highly effective at preventing infections, severe disease and death, and that breakthrough infections are uncommon.