Salient Features: Two weeks ago, the C.D.C. said Omicron accounted for just 1 percent of U.S. cases; a week ago, it was about 13 percent.

Delta, which for months had been the dominant form of the virus, accounted for about 26 percent of new cases over the last week, the C.D.C. estimated. Omicron, with more than 50 mutations, has turned out to be highly transmissible — two to three times as likely to spread as Delta and less susceptible to vaccines than other variants.