The highly contagious delta variant now accounts for more than 51% of COVID-19 cases in the U.S., according to new estimates released by the CDC. … And in parts of the U.S., the delta strain accounts for more than 80% of new infections, including some Midwestern states like Missouri, Kansas and Iowa. … The good news is the vaccines being used in the U.S. all appear to be highly effective at protecting against serious disease, hospitalization and death.