Most vaccines protect both the vaccinated individual and the society by reducing the transmission of infectious diseases. In order to eliminate infectious diseases, individuals need to consider social welfare beyond mere self-interest—regardless of ethnic, religious, or national group borders. It has therefore been proposed that vaccination poses a social contract in which individuals are morally obliged to get vaccinated. However, little is known about whether individuals indeed act upon this social contract. If so, vaccinated individuals should reciprocate by being more generous to a vaccinated other. On the contrary, if the other doesn’t vaccinate and violates the social contract, generosity should decline. Three preregistered experiments investigated how a person’s own vaccination behavior, others’ vaccination behavior, and others’ group membership influenced a person’s generosity toward respective others. The experiments consistently showed that especially compliant (i.e., vaccinated) individuals showed less generosity toward nonvaccinated individuals. This effect was independent of the others’ group membership, suggesting an unconditional moral principle. An internal metaanalysis (n = 1,032) confirmed the overall social contract effect. In a fourth experiment (n = 1,212), this pattern was especially pronounced among vaccinated individuals who perceived vaccination as a moral obligation. It is concluded that vaccination is a social contract in which cooperation is the morally right choice. Individuals act upon the social contract, and more so the stronger they perceive it as a moral obligation. Emphasizing the social contract could be a promising intervention to increase vaccine uptake, prevent free riding, and, eventually, support the elimination of infectious diseases.

大多数疫苗通过减少传染病的传播来保护接种疫苗的个人和社会。为了消除传染病，个人需要考虑超越自身利益的社会福利——无论种族、宗教或国家群体边界如何。因此，有人提出疫苗接种构成了一种社会契约，其中个人在道德上有义务接种疫苗。然而，人们对个人是否确实按照这种社会契约行事知之甚少。如果是这样，接种疫苗的人应该对接种疫苗的其他人更加慷慨作为回报。相反，如果对方不接种疫苗并违反社会契约，慷慨就应该下降。三项预先注册的实验调查了一个人自己的疫苗接种行为、他人的疫苗接种行为、和其他人的团体成员身份会影响一个人对其他人的慷慨程度。实验一致表明，特别顺从（即接种疫苗）的个体对未接种疫苗的个体表现出较少的慷慨。这种影响独立于其他人的团体成员身份，暗示了无条件的道德原则。内部荟萃分析（n = 1,032) 证实了整体社会契约效应。在第四个实验 ( n = 1,212) 中，这种模式在将疫苗接种视为道德义务的已接种疫苗的个体中尤为明显。结论是疫苗接种是一种社会契约，其中合作是道德上正确的选择。个人根据社会契约行事，他们越将其视为道德义务，就越是如此。强调社会契约可能是一种很有前景的干预措施，可以增加疫苗接种、防止搭便车，并最终支持消除传染病。