As the Omicron variant snowballs in South Africa and widens its inroads in Europe, evidence is mounting that it can outcompete the highly transmissible Delta variant — a potential warning signal for the United States. Viruses perform differently in different places, depending on what other variants are circulating and on what the landscape of immunity looks like. But Omicron is picking up speed in Europe, which has often served as a preview of what was headed the U.S.’s way.