Herd immunity is a term describing a steady-state distribution and one avenue (the typical one) of getting there is vaccines. Typically when 80% of the population\* is immune for one reason or the other a population is said to have herd immunity. The idea is statistical safety for susceptible individuals (yes, typically those that cannot be vaccinated for $reasons) thanks to being significantly outnumbered by those immune/resistant to infection - or a buffer population acting as a firewall, if you will.

I haven’t seen the term used for small pox in the Modern Era, but I believe it would be fair to say even though the small pox vaccine hadn’t been invented (and wouldn’t be until 1796), there were definitely periods of herd immunity in major European cities/ports of the time as it absolutely destroyed the previously unexposed Native American population.

But when dealing with “learned immunity” due to previous exposure as compared to natural/genetic immunity to the same diseases and the antibodies are not passed down from mother to child, what you’re going to see is a resurgence in cases as each new generation loses that immunity. In the present case, the unvaccinated herd immunity would be something to keep spread in check until a vaccinated herd immunity can take its place (assuming the vaccine isn’t developed before 80% of the remaining population has been exposed to and has recovered from this particular variant of the coronavirus).

\* at least that’s the number we were taught in my livestock/farm animals veterinary medicine class a decade ago.