Sunetra Gupta has written a thoughtful explanation of herd immunity.
It’s a riposte to a claim by the UK health minister that herd immunity is impossible for Covid19. In short she says individual immunity to Covid19 is unlikely to be permanent or complete, as with many similar viruses, so herd immunity is variable, but the level it occurs reduces widespread infection.
https://unherd.com/2020/10/matt-hancock-is-wrong-about-herd-immunity/