Dissertation Title:  "Economic Evaluation of Vaccination Policies Using Mathematical Models: Cost-Effectiveness, Externalities, and Affordability of Hepatitis B Vaccination"

Hepatitis B virus (HBV) infection remains a global public health challenge that causes significant morbidity and mortality from cirrhosis and primary liver cancer. Because the disease burden and epidemiological features of hepatitis B are disproportionately distributed globally, each region faces different policy questions in controlling HBV using vaccines. In industrialized countries, program efficiency and political commitments are of primary concern, while in low-income countries, financial sustainability must also be taken into consideration. This dissertation seeks to develop and apply various types of mathematical models to evaluate the cost-effectiveness, externalities, and affordability of different hepatitis B vaccination policies in two different settings: the U.S., a representative of low-endemic industrialized countries, and The Gambia, a representative of high-endemic lowest-income countries.

The first study evaluates the cost-effectiveness of hepatitis B vaccination strategies targeting potentially high-risk persons attending publicly funded HIV counseling and testing sites (CTSs) in the U.S. The results show that routine vaccination would be both more effective and cost-effective than screening strategies. The findings also imply that integrating a hepatitis B prevention service into existing HIV CTSs and routinely vaccinating high-risk adults would be a highly cost-effective public health intervention and may also have positive effects on the control of other types of viral diseases.

The second study develops a dynamic mathematical model in the form of a system of partial differential equations that can describe the transmission dynamics of HBV, incorporating heterogeneity in sex, age, and risk behaviors, and validates the model by comparing the predicted model outcomes with empirical data. Using the model, the study predicts the potential overall impact of the selective vaccination program identified as cost-effective in the first study on the control of HBV at the national level over time. Since programs for high-risk adults often suffer from a lack of political commitment to their implementation, the study also tries to quantify positive externalities that might result from the vaccination program. The study shows that a catch-up vaccination targeting high-risk adults would significantly reduce HBV incidence among those adults, accelerating the control of HBV in the U.S. The study also finds that vaccinating high-risk individuals would provide positive externalities to society, lowering HBV incidence even among unvaccinated low-risk individuals. About 30% of the cumulative number of HBV infections prevented due to the selective vaccination are estimated to come from externalities, and the externalities increase with vaccine coverage in a non-linear pattern.

The third study assesses the health impact, cost-effectiveness, and affordability of routine infant hepatitis B vaccination in The Gambia. Although The Gambia is receiving support for the vaccination program from the Global Alliance on Vaccines and Immunization, financial sustainability after the 5-year grant remains a big challenge to maintaining the program. The findings show that universal infant hepatitis B vaccination is highly cost-effective and has the potential to be affordable even with relatively low budgets in The Gambia. However, for the program to be affordable, the problem of narrowing the current funding gap awaits solution, and a full analysis of affordability requires further research.