Natural herd immunity is an utter fantasy. There has never been herd immunity for polio, measles, or even for chicken pox.

Herd immunity only happens when a critical mass of the population - >70% at a minimum - is given immunity by a vaccine.

This creates a population with a low enough R to guarantee that outbreaks die out, even if some people aren't vaccinated.

Even then it still doesn't guarantee personal immunity, because people in small clusters can still become infected. But instead of a national health emergency which threatens to overwhelm primary care, you get small localised individual/cluster outbreaks which are very much easier to handle.

That diseases come in waves seems like evidence of herd immunity to me. Like starting with a naive population you get a wave of infection. People become immune, spreading (mostly) stops. Over the years immunity is weakened because of old people dying and new people being born. Eventually the fraction of the population that is immune is low enough that a new wave gets going.

Absolutely right. People need to understand this.

> There has never been herd immunity for polio, measles, or even for chicken pox.

I think you've got a selection bias there. Those illnesses are household names because they are/were so prevalent. But what of the illnesses that never became prevalent enough to become household names because populations developed immunity fast enough to suppress the spread?

What's the last illness that never became prevalent enough to become a household name because populations developed immunity fast enough to suppress the spread?

I can't think of any.