Arguably, individuals vaccinate because they deem it in their interest to do so. But getting vaccinated comes at a cost. This cost can be monetary, psychological, logistical, or a matter of ideology. Importantly, such costs are not peculiar to the widely publicized anti-vaccinationists – going to a doctor can be an inconvenience to anyone. As for the (private) benefits of vaccination, they depend on the risk of being infected, that is on the prevalence of the disease in the whole population – if no one is infected, then there is no point in getting vaccinated.
Because of this, some individuals engage in free-riding by refusing vaccination. As vaccination coverage increases, unvaccinated individuals are less at risk of being infected. Free-riders who do not vaccinate do not incur their vaccination cost, but they still benefit from the effort of those who vaccinate. The opportunity to free-ride on others' vaccination makes vaccination decision a strategic interaction between individuals.
Is this a problem? Yes, because vaccination is not only about private incentives. Some individuals do not have access to vaccination for medical reasons, e.g. if they are too young or too sick to vaccinate. Only a high vaccination coverage in the population can protect them. Target vaccination coverages regularly fail to be met, and free-riding might well explain why.
There is a rich and still growing body of academic research on vaccination behavior in the field of biology, epidemiology, and game theory. However, including individual vaccination behavior in epidemiological models remains challenging: individuals base their vaccination decision on their anticipation of future prevalence, but this decision in turn shapes future prevalence. Nicolas Houy, Philippe Michel, and I, are developing general models of infectious diseases featuring individual anticipatory and strategic vaccination behavior. We produce numerical benchmarks, investigate anti-vaccination behavior, and apply our methods to the economic evaluation of public health policies.
Yet the implications of our work go beyond the issue of vaccination behavior. We use the example of vaccination policies to illustrate how individual responses to a policy may determine its success or failure. We show that if individuals anticipate the effects of such policies and behave strategically, the policies might well backfire and produce unwanted effects that cannot be overlooked.