Earlier quoted context omitted.
The problem with herd immunity is that the rate differs based on the variant. For something much more transmissible (like the Delta variant), herd immunity numbers are much different than for the original one. There's an argument that the US reached herd immunity on the original one pretty early on in vaccinations. And alpha with current vaccination rates. It's clear we haven't with Delta because numbers are going wa…
Immunity to one would be immunity to the other They use the same vector for infection which so the immunities offer protection against. The protection mechanism is a game of catch up though.
Herd immunity is when enough people have individual immunity that when those without immunity get infected so many of those around them are immune that the outbreak quickly dies out on its own.
Even if individual immunity is the same for variants A and B of some virus, if variant B spreads more easily than variant A then you will have to have a higher percentage of the population with individual immunity in order to have herd immunity.