Making vaccine refusal less of a free ride.

Herd immunity against vaccine-preventable diseases is a public good because it is both non-excludable (meaning that there is no way to exclude people from using it) and non-rivalrous (meaning that one person's use does not limit or restrict others' use). Like other public goods, such as lighthouses, street lights and national defense, herd immunity is vulnerable to the "free rider" problem. We discuss four conventional responses to the free rider problem (participation mandates, exclusion, incentives, and social norms) and highlight how a public good perspective can inform the design of interventions to increase vaccine acceptance.