[Question] By which mechanism does immunity favor new Covid variants?

I’m not a biologist, but I’m trying to get something like a gears-level understanding of the situation we are in. There are phenomena that seem superficially clear to me, but if I’m honest, I do not really understand what’s going on.

For example, take the new Covid variants from Great Britain, South Africa and India. It is often said that the danger for such mutations is higher if a population already has some immunity because it increases the selection pressure. First, I thought “that’s obvious, it’s just how evolution works”. But, how exactly does this work?

Here is my current understanding:
* Viruses do not have a metabolism, which is why I suspect that there is no strong competition in the human body among different virus variants. (Please correct me if this is wrong—I asked some friends doing biology but they also could not tell me whether this is true.)
* Immunity essentially disadvantages the “old” variant, but because of the first point, it does not give any advantage to the “new” variants.
* Although the share of the more contagious, new variants are higher with some existing immunity, in absolute numbers, the new variants are not more successful than without immunity.
* It even seems plausible (and for some variants, there’s evidence) that immunity agains the old variant also gives partial immunity agains the new variants.
* I conclude that the presence of the new variants is mainly driven by the mere number of viruses, which is proportional to the number of mutations. When the number of viruses increases, the chance of a dangerous mutation also increases. However, immunity seems to be a good thing to mitigate mutations in every case.

Unfortunately, I must have made a mistake somewhere because experts say otherwise.
I’d be grateful for suggestions how this mechanism “immunity → selection pressure → more creepy variants” really works!

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