The Delta variant of the novel coronavirus, first found in India, is the greatest threat to the United States' effort to eradicate COVID-19 in its borders, said U.S. 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."