Aetiology: Another mumps post--isn't the vaccine working?

Yes, there's disease in the vaccinated population. Of the cases in this little hypothetical, by the numbers alone, 66% were vaccinated--lower, but similar to our numbers here in Iowa. Yet as you can see, this doesn't mean that "the vaccine isn't working:" in our scenario, it means it's working at a 90% effectiveness rate, which is pretty good. The unvaccinated population acquired disease at 10 times the rate of the vaccinated population overall, so while being vaccinated was no guarantee of protection, it's a damn good gamble.