An effective influenza vaccination policy is fundamental, not only to reduce indivdual's risk of infection with influenza, but also to prevent population epidemics as well. Recent outbreaks of A (H1N1) influenza beginning in April 2009 have reminded us of the importance of reconsidering policies that allows successful control of future influenza pandemics. Quantitative evaluation of vaccination strategies based on scientific analysis is essential. Important policy questions include who should be prioritized for influenza vaccination and how widely influenza vaccination should cover in the population. We propose to evaluate the cost-effectiveness of influenza vaccination policies to address these questions in the US and in Japan. We will use a mathematical model that describes disease transmission to evaluate cost and effectiveness of vaccination strategies. The model will be parameterized by country-specific data, which will be collected by national representative surveys as well as a series of literature. We have two specific research questions. First, we will analyze how differences in influenza transmission between the US and Japan affect cost and effectiveness of influenza vaccination in both countries. Epidemiological, demographic and socio-economic characteristics differ between the contries, and the difference should be reflected by vaccination policies. For example, Japan is a country with a high population density and its demography is characterized as an aging population. Second, we evaluate cost-effectiveness of several proposed strategies as a comparison with the current vaccination policies, recommended by the Center for Disease Control and Prevention in the United States and by the Ministry of Health Labor and Welfare in Japan. We will evaluate alternative strategies suggested by authorities and/or researchers as a comparison with the current recommendation policies. The alternative strategies include the universal vaccination plan among children and the recommendation by HealthyPeople2010 that prioritized the elderly. To answer these research questions, it is essential to use a mathematical model of epidemiology to characterize the dynamics of disease transmission. Vaccination produces an indirect benefit to non-vaccinated individuals in the same community as well, and the indirect benefits of vaccination should be taken into account in policy evaluation. It is also important to use an age-structured model as transmission rate, efficacy of vaccine, and infection severity are age-dependent. For example, it is widely recognized that the transmission of influenza is mostly attributed to schoolchildren, while the largest cost from influenza infection falls on those at high risk. Evaluating the cost and effectiveness of influenza vaccination program has policy implications. At the same time, this comparative approach would also provide an important insight in the impact of epidemiological and socio-demographic differences on cost-effectiveness analyses.