You raised a good point there that I missed in this article. An immunocompromised state is, indeed, the more well-known selection pressure for viral evolution of immune-escape mutants. And population density is also a crucial factor for viral evolution, as you and the other commenter have pointed out. I guess there are many things to consider for viral evolution, some of which I must admit that I don't fully comprehend.

I also find it odd that Delta doesn't have the E484K mutation, but I didn't probe much further. I also agree that Delta might gain the E484K mutation in the future as the population's immunity against SARS-CoV-2 increases. Anyway, your comment is an intelligent one, so thank you for sharing.

If you ever write an article about this, I'd be more than happy to link it in this article as an add-on. :))