The Delta variant of the novel coronavirus that was first found in India is the greatest threat to the United States' effort to eradicate COVID-19 in its borders, said US infectious disease expert Dr Anthony Fauci during a Tuesday press call.

"The transmissibility is unquestionably greater" in the Delta variant than the original variant of COVID-19, Fauci said, adding that "it is associated with an increased disease severity."

The Delta variant is becoming the globally dominant version of the disease, the World Health Organization's chief scientist said on Friday.

Fauci said that the vaccines authorized in the United States, including the Pfizer/BioNTech vaccine, are effective against the new variant of COVID-19.

"We have the tools, so let's use them and crush the outbreak," Fauci said.

The United States is falling short of its goal of vaccinating 70% of adults by July 4 and will likely need a few additional weeks to hit that target, White House COVID-19 senior adviser Jeffrey Zients said during the press call.

Zients added that he expects 70% of adults over 27 years old will have had at least one shot by July 4.

More than 150 million people in the United States, or more than 45% of the population, have been fully vaccinated, according to federal data last updated on Monday.

The Delta variant contributed to a severe outbreak of COVID-19 in India during April and May that overwhelmed health services in the country and killed hundreds of thousands.