There is no compelling science to indicate that measles vaccines routinely mutate.
You think it matters whether it routinely mutates or mutates occasionally?
The parent is acting in good faith because they have no compelling reason to believe that the vaccine will spontaneously mutate into something else.
If a doctor told them it is possible, and it is, they would have a compelling reason to believe it's possible. If they had studied the question prior to consenting to the infection with a vaccine-strain virus, they would have found some compelling reasons.
They are acting based on the best medical science currently has to offer.
That's a pointless cliché.
What is the probability that any given vaccination has mutated into a virulent strain? Simple enough question.
Too bad you don't have an answer and are too lazy to research and find out. While you get to work on answering that, consider that some of us know people who developed clinical measles from the vaccination and then spread it to other people.
I don't support forced vaccination.
Make up your minds. You don't support it generally, but you do support it for certain diseases.
Nor do I have a problem excluding your child from certain activities if they are not vaccinated.
In other words, you have no problem persecuting my children for no good reason.
Will you forbid them from using public transportation, too? Yes? No? Why? Why not?