Disneyland measles outbreak: Cases not limited to the unvaccinated

"...Their cases point to a lesser-known aspect of the measles vaccine: That even those who get the shots have a small risk of getting sick, especially older people who were immunized in the 1960s, '70s and '80s..."
http://www.latimes.com/local/california/la-me-measles-spread-20150122-story.html