Let’s see. A virus encounters a barrier to survival, what does it do? It mutates.
And, this is the process of any other biological entity, except perhaps a snowflake — they just whine about it being unfair.
It is amazing the number of supposedly informed folk who don’t even try to understand basic natural selection. If you put non-lethal pressure on a system the inhabitants will live/die until one emerges to resist the pressure. The more pressure, the faster the process. More agents acting to increase the pressure.....
Note: non-lethal pressure. Until the CDC re-defined “vaccine” to specifically include mRNA tech all vaccines were not one unless they killed the bug.
Viruses are not thinking creatures. They all mutate. When there is nothing in the body to do away with the original virus, they have a clear field to multiply and mutate until one finally mutates into a more contagious or more severe type. Voila! Variant!
All evolution follows the same basic rules.
Mixture, mutation, natural selection, and random genetic drift.