It seems to turn out that in a recent measles epidemic (by modern standards) in NYC, “patient zero” was, indeed, fully vaccinated. And some of the folk she infected were vaccinated or previously exposed as well.
So yup, vaccines aren’t perfect and neither are human antibodies. But it would have been a lot worse if more people would have been susceptible (that is, unvaccinated):
“Results. The index case had two doses of measles-containing vaccine. Of 88 contacts, four secondary cases were confirmed that had either two doses of measles-containing vaccine or a past positive measles IgG antibody”