vaccinated (free-riding); they may also be influenced to vaccinate by the fact that vaccination would protect others (altruism). Other factors include perceptions of disease risk and the ability to control those risks, and preferences for the risks of diseases per se over risks of the vaccine against them. Studies have also addressed issues of mandatory vaccination, informed consent, individual rights versus societal welfare, and people's trust in information providers.

A major influence on the acceptance of vaccine risks is whether people employ what is known in the risk communication field as bandwagoning, free-riding, or altruistic logic. Bandwagoning refers to the tendency for individuals to choose the decision of the majority as an indication of what might be a wise action for themselves, without fully evaluating their options. A study at the University of Pennsylvania found, for example, that when parents who vaccinate their children were given a hypothetical situation in which 100 percent of other children were vaccinated for a particular disease, 95 percent said that they would also vaccinate even though their children would be at no risk of catching the disease (Meszaros et al., 1996).

The tendency to bandwagon is countered by free-riding logic. People who follow this logic feel that they do not have to expose themselves to the risks of vaccination because they are protected from disease by the vaccination of the majority of other people (a phenomenon known as herd immunity). Nonvaccinators in the University of Pennsylvania study were more likely to use free-riding logic than were vaccinators. People who use altruistic logic, in contrast, are willing to take on personal risks if a large number of people will benefit by their doing so. Overall, bandwagoning appears to be much more common than either altruism or free-riding (Hershey et al., 1994).

Perception of the risk of contracting a disease influences willingness to accept the risk of a vaccination. For example, a common misconception among parents in one Washington, D.C. survey is that vaccine-preventable diseases have been virtually eliminated in the United States, thus ending the need to immunize children (McLaren, unpublished data presented at the workshop). The success of vaccination programs in reducing the incidence of vaccine-preventable diseases in the United States makes it more difficult to communicate the risk of those diseases.