An overwhelming majority of new COVID-19 cases have been caused by the Delta variant, the Centers for Disease Control and Prevention said Tuesday. CDC Director Rochelle Walensky said the Delta variant was responsible for 83 percent of new COVID-19 cases. “This is a dramatic increase, up from 50 percent for the week of July 3,” Walensky said at a Senate committee hearing. It comes as data from Israel suggests the Pfizer vaccine is 64 percent effective at preventing infection from the Delta COVID-19 variant and 93 percent effective at preventing serious illness and death.

The largest issue, according to the CDC, is the number of people still unvaccinated: about half of Americans, with vaccine hesitancy still prevalent. “There have been multiple times when we have been fooled by COVID-19, when cases went down and we thought we were in the clear and then cases went up again,” Surgeon General Vivek Murthy told CNN. “It means we shouldn’t let down our guard until cases not only come down but stay down, and right now cases are actually going up.”