The Delta variant of the coronavirus is the "greatest threat" to eradicating Covid-19 in the U.S., according to Anthony Fauci, the country's leading infectious disease expert.

The variant, first discovered in India, is more contagious and causes more serious disease than other circulating variants. The lineage spread quickly in some sections of the country and demonstrated partial resistance to vaccines.

However, researchers found it impossible to separate the variant's inherent features from other variables propelling India's confirmed cases past 400,000 per day, such as mass gatherings.

Delta cases in the U.S. have now more than doubled in just two weeks. The variant was responsible for more than 20% of new cases as of June 19, up from roughly 10% the week ending June 5.

The Centers for Disease Control and Prevention has already classified Delta as a "variant of concern."

Over the course of two months, meanwhile, the Delta variant surpassed the Alpha, or B.1.1.7, variant in the U.K. Delta cases now account for "well over 95%" of new cases in the U.K., according to Fauci.

In terms of variants, the U.S. "has followed the U.K."

But rates are lower among young adults ages 18 to 26, he said, and the U.S. will likely fall short of President Joe Biden's goal of partially vaccinating 70% of all adults by Independence Day.

"With the Delta variant now spreading across the country, and infecting younger people worldwide, it's more important than ever that they get vaccinated," Jeffrey Zients, the head of the White House Covid-19 response team, said.

The good news, according to Fauci, is that vaccines approved in the United States, notably the Pfizer/BioNTech vaccine, are effective against the new variant of COVID-19.

According to federal statistics last updated on Monday, more than 150 million people in the U.S., or more than 45%of the population, have had full vaccinations.