And this is really interesting. I've just had a look at the dept of health info on mumps. Anyway they actually state it's a mild disease etc, rarely causes complication, infertility extremely rare. They state that the vaccine is 95% effective ()based on a 1999 paper) but note that later papers (they lost a 2004 one) found it to be not as effective (they don't give efficacy though)
However on Pulse ('the UK's leading medical journal' according to their blurb- read by 80% of GPs) I found this from 2005:
"27 May 05
A 'substantial' proportion of mumps cases are occurring among children who have received a single dose of the MMR vaccine, Health Protection Agency scientists report.
The HPA is warning GPs to ensure all at-risk patients receive both doses of the vaccine after its study suggested a single dose was insufficient to provide protection.
The HPA researchers, who presented their data at the European Society for Paediatric Infectious Diseases conference in Valencia last week, suggested 'the efficacy of the mumps component of MMR needs further investigation'.
The study reviewed cases of mumps in 2004, when 47 per cent occurred among people aged 15 to 19, who had received or were eligible for a single dose of MMR.
Researchers used immunological modelling to estimate a 'high proportion' of them were weak positives, meaning they had received the vaccine but had low levels of antibodies.
Dr David Elliman, consultant in community child health at Great Ormond Street Hospital, said: 'As this paper suggests, perhaps the protection against mumps is not as good as we had thought.'
Dr George Kassianos, RCGP immunisation spokesperson and a GP in Bracknell, Berkshire, said the rate of seroconversion to the mumps component 'may be much lower than we have thought in the past  a second dose of the MMR vaccine is even more important'."
This is important because of the risks of pushing mumps into an older age group (because the vaccine doesn't work as well or for as long as estimated during the very short trials) and thus making it a more dangerous disease.