[mywittyname]

It has been demonstrated that repeated infections are possible and may be common. And there are a large number of variants in the wild that are not hindered by immune systems previously exposed to other variants. Allowing the virus to spread in populations causes even more variants to emerge.

Even vaccines provide reduced protection from new variants. I read today that most effective vaccine only provides a ~70% chance or so of protection from the Delta variant in the UK. Hence the talk of booster shots before the initial vaccines were rolled out.

Herd immunity comes from vaccination. It's not really feasible for it to occur naturally because the virus will continue to mutate and reinfect people indefinitely.