Arguably, individuals vaccinate because they deem it in their interest to do so. Getting vaccinated comes at a cost. This cost can be monetary, psychological, logistical, or a matter of ideology. Importantly, such costs are not unique 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, there is no point getting vaccinated.
Because of this, some individuals engage in free-riding and refuse vaccination. As vaccination coverage increases, unvaccinated individuals are less at risk of being infected. Free-riders do not incur their vaccination cost, but they still benefit from the effort of those who vaccinate. 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 coverage 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 behaviour in the field of biology, epidemiology, and game theory. However, including individual vaccination behaviour in epidemiological models remains challenging: individuals base their decision on their anticipation of future prevalence, but their decision in turn shapes the future. Nicolas Houy, Philippe Michel, and I, are developing general models of infectious diseases featuring individual anticipatory and strategic vaccination behaviour. We produce numerical benchmarks, investigate anti-vaccination behaviour, and apply our methods to the economic evaluation of public health policies.