Delta variant 'greatest threat' in U.S. COVID-19 fight -Fauci

"Good news: our vaccines are effective against the Delta variant," Fauci added.

"This variant represents a set of mutations that could lead to future mutations that evade our vaccine. And that's why it's more important than ever to get vaccinated now, to stop the chain of infection - the chain of mutations that could lead to a more dangerous variant," said Rochelle Walensky, the director of the U.S. Centers for Disease Control and Prevention (CDC).

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 has killed hundreds of thousands.