Well the authors state the answer is herd immunity. All of those immunized individuals are beginning to provide protection for even those who have not been immunized as these strains of the virus (6,11,16,18 and maybe some of types 31, 33, 45, 52 and 58 ) are not able to effectively spread through the population like they once could.

Where's the evidence?

Although the evidence for vaccination-based herd immunity is yet to materialize, there is plenty of evidence to the contrary. Just a single publication by Poland & Jacobson (1994) reports on 18 different measles outbreaks throughout North America, occurring in school populations with very-high vaccination coverage for measles (71% to 99.8%). In these outbreaks, vaccinated children constituted 30% to 100% of measles cases. Many more similar outbreaks, occurring after 1994, can be found by searching epidemiologic literature.
Source: http://www.greenmedinfo.com/blog/herd-immunity-myth-or-reality

And
Why Is China Having Measles Outbreaks When 99% Are Vaccinated?