The Delta variant makes up an estimated 83 percent of U.S. cases, the C.D.C. director says.

The highly infectious Delta variant now accounts for an estimated 83 percent of new coronavirus cases in the United States — a “dramatic increase” from early July, when it crossed the 50 percent threshold to become the dominant variant in this country, the director of the Centers for Disease Control and Prevention said Tuesday.

In some regions, the percentage is even higher — particularly where vaccination rates are low, Dr. Rochelle Walensky, the C.D.C. director, said during a Senate health committee hearing. Vaccines are effective against the Delta variant and while almost 60 percent of U.S. adults are fully vaccinated, less than half of the total U.S. population is.

She said the C.D.C. would update its website later Tuesday to reflect the new estimate of Delta cases, which the agency derives from gene sequencing of new coronavirus cases.

The new figure comes as new cases have been rising across the United States, though cases, hospitalizations and deaths remain a fraction of their peaks. Still, public health experts are watching the increases with deep concern and Dr. Walensky warned last week that “this is becoming a pandemic of the unvaccinated.” The seven-day average now shows more than 35,000 new daily cases, up from about 11,000 a day not long ago, according to a New York Times database.

Tuesday’s hearing was contentious at times. Senator Mitt Romney, Republican of Utah, pressed Dr. Janet Woodcock, the acting commissioner of the Food and Drug Administration, on when the F.D.A. would authorize booster shots — and was not happy when she could not provide a specific answer. Federal health officials have said booster shots are not necessary now and have pressed Pfizer for more evidence.

Other Republicans clashed with witnesses over matters including mask mandates, booster shots for Covid-19 vaccines and “gain of function” research designed to identify genetic mutations that could make a virus more powerful.

Senator Rand Paul, Republican of Kentucky, escalated his long-running attacks on Dr. Anthony S. Fauci, President Biden’s top medical adviser for the coronavirus pandemic, and accused Dr. Fauci of committing a crime by lying to Congress in May when he told senators that the National Institutes of Health did not fund “gain of function” research at a laboratory in Wuhan, China, the epicenter of the pandemic’s early days.

Dr. Fauci, in turn, accused the senator of falsely implying that the N.I.H. is somehow responsible for hundreds of thousands of deaths from the pandemic — an extraordinary exchange for the Senate, where witnesses almost always defer to lawmakers.

“I have never lied before Congress and I do not retract that statement,” Dr. Fauci declared, adding, “Senator Paul, you do not know what you are talking about, quite frankly, and I want to say that officially.”