In community-wide immunisation programmes against childhood infections there is a conflict between the interests of the individual (vaccine safety and efficacy) and the interests of the community (vaccine uptake and level of herd immunity). Studies suggesting that the complication rate is greater with the high efficacy Urabe Am 9 mumps vaccine than with the lower efficacy Jeryl Lynn vaccine, have led to concern about whether the higher efficacy mumps vaccine should be introduced or retained in nationwide mass immunisation programmes. We describe the use of a mathematical model to assess benefits and risks to both individual and community, and illustrate this method by reference to immunisation programmes based on these vaccines. On the basis of current epidemiological data on viral transmission and vaccine coverage in England and Wales, data on vaccine-associated and infection-associated complication rates, and vaccine efficacies estimated from clinical trials, our analyses suggest there is little to choose between the two vaccines, but that overall performance depends on the level of vaccine coverage in a defined population. In community-based programmes, the greater apparent safety of the Jeryl Lynn vaccine (fewer vaccine-induced complications) is offset by the greater apparent efficacy of the Urabe Am 9 vaccine (fewer complications due to natural infection). The findings suggest that it may not always be in the interests of the community to use the vaccine with the lowest complication rate.