Nearly all new coronavirus cases, 83 percent of them, in the United States are a result of the highly infectious Delta variant. This increase is especially concerning because of how quickly the variant has come to dominate new cases. In early July, it crossed the 50% threshold to become the most common variant in this country.

In areas with low vaccination rates, the percentage of Delta variant cases remains high, C.D.C. Director Dr. Rochelle Walensky told the Senate health committee. Today, even though nearly 60 percent of U.S. adults are fully vaccinated and vaccines remain effective against the Delta variant, the majority of the population remains unvaccinated.

Dr. Rochelle Walensky confirmed that the C.D.C. would update its website this week to reflect the current rate of cases consisting of the Delta variant. The organization determines this estimate from gene sequencing of new coronavirus cases.

Although cases, hospitalizations and deaths are down compared to their peaks, these new cases have been rising across the country. These increases have brought deep concern to public health officials, as Dr. Walensky points out that “this is becoming a pandemic of the unvaccinated.” One database from the New York Times displays these concerns, with weekly case averages rising from 11,000 daily cases a few days ago to 38,000 daily cases.