My understanding is that "herd immunity" is only achieved with the use of an effective vaccine.

Is that correct?

ETA: Doh, of course there has. I think I was confusing something, perhaps semantics.... nevermind.

ETA2: Now I know what I was thinking of. From James Hamblin, discussing whether "herd immunity strategy" was a thing.

"It is the concept used in vaccination policy, to calculate how and when vaccines need to be deployed in order to prevent outbreak of a disease."