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Volume 17,     Number 2,     Summer 2009

 

THE FREE RIDER PROBLEM IN VACCINATION
POLICY AND IMPLICATIONS FOR GLOBAL
ERADICATION OF INFECTIOUS DISEASES:
A TWO-COUNTRY GAME DYNAMIC MODEL
LETA MONTOPOLI, SAMIT BHATTACHARYYA
AND CHRIS T. BAUCH

Abstract. Increasingly, an important obstacle to local elimination of vaccine preventable infectious disease is vaccine exemption by individuals under a voluntary vaccination policy, caused by free-riding on herd immunity or by inflated perceptions of vaccine risk. At a global level, this can also prevent global eradication of an infection, since any one country with endemic infection can act as a reservoir, seeding the rest of the world with infections. This also presents challenges to non-governmental third parties that fund and support global eradication efforts through vaccination campaigns, who must choose how to optimally deploy their funds to achieve global eradication in the face of vaccine exemption. Behaviour-prevalence models capture the interplay between infection prevalence and disease transmission and have been applied to study free-riding behaviour in vaccination policy before. However, these obstacles to global eradication require a multi-country behaviour-prevalence model that incorporates travel between countries and also incentives by governments and nongovernmental donors. Here, we develop a two-country model based on imitation dynamics for a pediatric infectious disease. We analyze the model numerically, showing a range of possible dynamic behaviours such as sustained oscillations in infection prevalence and the abundance of vaccinator strategists, and also parameter regimes where a certain budget allocation by the third party enables elimination in both countries. Similar models may be useful in the future as planning tools for international organizations working on the global eradication of certain infectious diseases, such as measles and polio.

 

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