The goal is herd immunity against COVID-19, when will we know if we have reached it?

Proof that we’re nearing herd immunity would be a “disruption in the chain of transmission.”

from http://www.firstpost.com/tech/science/the-goal-is-herd-immunity-against-covid-19-when-will-we-know-if-we-have-reached-it-9318011.html