But that's what I mean, viruses need to replicate. They do this by injecting their RNA into your cells and hijacking your ribosomes for their replication. So the first viruses would definitely get past your immune system, but the replicated viruses would then be produced by your cells, so they wouldn't have the U->psi replacement that the first generation had. So every subsequent generation of the virus could be fought by your immune system. Effectively giving the virus a head start of one replication cycle. I'm guessing this wouldn't change much. But I'm not a biologist.