It doesn't. The research says the vaccine's been so effective that some people lose the immunity before they even use it. Case in point, the original case in question had been vaccinated decades prior and spent her time surrounded by tourists with all sorts of different kinds of measles. Then she'd spread it to some others; some of which also lost most of their immunity. They didn't spread measles at all, and the secondary infection was so mild the only reason anyone knew it was measles was because the CDC was already investigating for it .

TL;DR: The measles vaccine has been so effective it's actually slightly counter-productive to some.