C Roberts, G Porter-Jones, J Crocker, J Hart. Mumps outbreak on the island of Anglesey, North Wales, December 2008-January 2009. Eurosurveillance, Volume 14, Issue 5, 05 February 2009. Euro Surveill. 2009;14(5):pii=19109. Available online

Twenty-three cases of clinical mumps in young people have been reported in North Wales over a five-week period since late December 2008. All cases have social links, and most of them have received two doses of mumps-containing vaccine.

And:

...An unusual feature of this outbreak is that 20 of the cases had received two doses of the measles, mumps, rubella (MMR) vaccine and two cases had had one dose. The only unvaccinated case was a 37 year-old patient who was too old to have been offered MMR as a child. ...

And in the discussion they state, making a claim that unvaccinated people did not get the mumps because of the high uptake of vaccine:

Uptake of MMR vaccination has historically been high in Anglesey, and the majority of cases in the outbreak had received two doses. The lack of cases among unvaccinated individuals may reflect the high uptake of vaccine, and an investigation is ongoing to determine coverage rates for the birth cohorts involved.

If that really was true, then it should have been the unvaccinated people that should have got the mumps, not the other way around.