What makes the COVID-19 variant different, and dangerous The CDC estimates the delta variant of the coronavirus that causes COVID-19 is responsible for more than 82 percent of cases in the U.S. Author: A Department of Energy researcher at the Oak Ridge National Laboratory studies the coronavirus in this photo from June 2020. Since then, the COVID-19 virus has mutated into several variants and the latest, the delta variant, has been called the “fastest and fittest” for its high transmissibility rate. (Photo by Genevieve Martin/U.S. Department of Energy) The CDC estimates the delta variant of the coronavirus that causes COVID-19 is responsible for more than 82 percent of cases in the U.S.