Health officials say the delta variant of the coronavirus continues to surge and accounts for an estimated 83% of U.S. COVID-19 cases.

That’s a dramatic increase from the week of July 3, when the variant accounted for about 50% of genetically sequenced coronavirus cases.

“The best way to prevent the spread of COVID-19 variants is to prevent the spread of disease, and vaccination is the most powerful tool we have,” said Dr. Rochelle Walensky, director if the Centers for Disease Control and Prevention, during a U.S. Senate hearing Tuesday.

The delta variant is a mutated coronavirus that spreads more easily than other versions. It was first detected in India but now has been identified around the world.