The catch is, no one even knows for certain what the threshold of herd immunity for Covid-19 is, Dr.

"When you get that level of protection that turns out to be the threshold of herd immunity, you'll know it, because you'll see the infections almost disappear," Fauci told Varshavski.

"Forget this issue of herd immunity and just get as many people vaccinated as you possibly can, as quickly as you possibly can" to slow transmission of the virus, he told The New York Times' Apoorva Mandavilli on "The Daily" May 7.