The Delta variant now accounts for an estimated 93.4 per cent of all the new Covid-19 cases in the US that were reported during the last two weeks of July, according to the US Centers for Disease Control and Prevention on Wednesday.
In its latest update on Wednesday, the CDC said that this number is even higher in certain parts of the country, showed the updated figures for the week ending July 31, reports Xinhua news agency.
In the Midwest including Iowa, Kansas, Missouri and Nebraska, Delta variant accounts for more than 98 per cent of all new cases.
These figures have shown a rapid increase over the past two months. In the two weeks ending May 22, Delta's prevalence was estimated around 3 per cent, said the CDC update.
The daily average of new cases over the past week rose to 66,606 per day, from 40,597 the previous week, an increase of more than 64 per cent, it added.
In an interview on Wednesday, Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said that the cases driven by the Delta variant may double to 200,000 a day in the fall.
"Remember, just a couple of months ago, we were having about 10,000 cases a day. I think you're likely going to wind up somewhere between 100,000 and 200,000 cases," he said.
The cases driven by the Delta variant are rising in a "very steep fashion" across the US and the country could be "in trouble" entering the fall unless a large portion of unvaccinated Americans decide to get the shots, said Fauci.
"What we're seeing, because of this increase in transmissibility, and because we have about 93 million people in this country who are eligible to get vaccinated who don't get vaccinated - that you have a significant pool of vulnerable people," he added.
The top infectious disease expert said he is concerned the high number of unvaccinated people could lead to a stronger variant emerging that could combat the vaccines that have been given out.
"If we don't crush the outbreak to the point of getting the overwhelming proportion of the population vaccinated, then what will happen is the virus will continue to smolder through the fall into the winter, giving it ample chance to get a variant," Fauci said.
Since the onset of the pandemic early last year, the US has so far reported a total of 35,331,699 coronavirus cases and 614,803 deaths.
The two figure are the highest in the world, making the US the worst-hit country.