[Fomite]

Vaccine escape, which is the phenomena you're talking about, is extremely rare, and as far as I'm aware has never been observed in most vaccine preventable diseases.

The exception is perhaps influenza, but that's mainly because the flu vaccine is one of the few "moving target" viruses we have a vaccine against.

[leterter]

We are seeing it with HIV.

[Fomite]

HIV, which is admittedly another "moving target", isn't that way because of artificial selective pressure because of vaccines.

It's because RNA transcription doesn't have error checking.

Nor does an unstable antigen profile (ala HIV or influenza) necessarily imply more severe disease.