[Vecr]

Measles can't create an escape mutant, or at least it's so hard that centuries of immunity exerting pressure and gain of function experiments can't produce one. Flu recombines and reassorts like crazy. It has RNA segments (the media likes to call them "chromosomes", but they are not) that are pretty much interchangeable, so if you or a non-human animal gets infected with multiple strains, you can get a virus with a hybrid of those segments quite often. I think rubella actually can escape. The chickenpox vaccine mostly just suppresses symptoms. The smallpox vaccine is just another orthopoxvirus, so you get pretty good cross reactive immunity.