The Delta variant of coronavirus found in India has proved to be the greatest threat to the efforts made by the United States to fight Covid-19 in its borders, top scientist and US infectious disease expert Dr Anthony Fauci said on Tuesday. Spotlighting the highly transmissible strain, Fauci said the Delta variant is more easily transmitted than the original Covid-19 variant. On Friday, the World Health Organisation’s chief scientist said that the variant is becoming the globally dominant version of the disease.