Herd immunity means two different things, and we need to be clear on the difference.
In the old days, it meant that the majority of the population had had the disease and was, therefore, immune and wouldn't get it again. Mothers passed temporary immunity to their babes, limiting the incidence of some illnesses in very early life, children caught most everything during childhood and by the time they grew up were immune to most diseases. Mortality to these diseases was a result, not of the inherent viciousness of the disease, but the challenges of the human social and ecological environment, which consigned a large portion of the population to horrendous living conditions.
The other herd immunity is the belief that by vaccinating, the circulation of particular illnesses can be stopped, thereby increasing the sum of human health and happiness. This overlooks the substitution factor (block one illness, another will fill the niche), the increasing age factor (children don't get the diseases, teenagers and adults do), the loss of protection of infants and so forth. Plus the fact that some vaxes seem to spread the disease, others seem to undermine the immune system, and they all contain toxic substances, lessening the overall health of the population. Some herd immunity!
Whoops, forgot to mention the "booster" effect from circulating disease. One reason adult immunity held up was regular exposure from children with the illness. Reminded the body of the illness, thereby reinforcing the immune response (or something like that).
Deborah