If you re-read the article, you will notice that the 66% figure is specifically for those in Iowa who got mumps, and had been vaccinated. It then goes on to say "'It's working at a 95 percent efficacy rate, which is darn good,' Ms. Quinlisk said of the vaccine, which is required of school-age children.". So, my initial statement that the MMR is 95% effective is still correct; in the case of the Iowa mumps breakout, the 66% is an exception.
"'It's working at a 95 percent efficacy rate, which is darn good,' Ms. Quinlisk said of the vaccine"
I was responding to the fact that she said "it's working at a 95% rate", implying that it is working at that rate in Iowa, whereas it's not really working at 95% in Iowa.