Often, vaccination programs are carried out based on self-interest ratherthan being mandatory. Owing to the perceptions about risks associated withvaccines and the `herd immunity' effect, it may provide suboptimal vaccinationcoverage for the population as a whole. In this case, some subsidy policies maybe offered by the government to promote vaccination coverage. But, not allsubsidy policies are effective in controlling the transmission of infectiousdiseases. We address the question of which subsidy policy is best, and how toappropriately distribute the limited subsidies to maximize vaccine coverage. Toanswer these questions, we establish a model based on evolutionary game theory,where individuals try to maximize their personal payoffs when considering thevoluntary vaccination mechanism. Our model shows that voluntary vaccinationalone is insufficient to control an epidemic. Hence, two subsidy policies aresystematically studied: (1) in the free subsidy policy the total amount ofsubsidies is distributed to some individuals and all the donees may vaccinateat no cost, and (2) in the part-offset subsidy policy each vaccinated person isoffset by a certain proportion of the vaccination cost. Simulations suggestthat, since the part-offset subsidy policy can encourage more individuals to bevaccinated, the performance of this policy is significantly better than that ofthe free subsidy policy.
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