>You've just destroyed your own argument for herd immunity.

According to an internet rando with no medical qualifications. What shape do you suppose the Earth is, Cletus? 

>Huge numbers of adults who have been vaccinated are no longer immune because vaccines don't work for very long

Because viruses mutate. Do you acknowledge that viruses mutate?

>If herd immunity could be achieved...that would be by the population developing true, natural immunity, thereby limiting the potential for contagion and offering some protection to those potentially more vulnerable.

Viruses, bacteria and other pathogens mutate too quickly for this to occur. The rate of human evolution is drastically slower because a human takes over a decade to mature before they can reproduce. Microorganisms reproduce much, much, much faster.