A lot of people seem to think 'Herd Immunity'(tm) is somehow necessarily immunity without a vaccine (desirable for people somehow afraid of the vaccine).

It's really of course just a probability mechanism when immunity from whatever cause -- antibodies triggered by vaccine or disease -- becomes dominantly likely in the group(herd) population, the disease dies out.

From a Mayo Clinic info page.

Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune.