Economic evaluations of vaccines in Canada: Exploring the epidemiologic and economic impact of chickenpox vaccine strategies in Canada using an agent-based model

Abstract

Health economic evaluations are a systematic method of measuring and valuing the costs and effects of different health interventions. The cost-effectiveness of vaccines is particularly difficult to measure, as they are generally complicated by the infectious nature of the diseases. Using a scoping review framework, we gathered, summarised and described the evolution of published economic evaluations of vaccines in Canada. In recent years there has been a consistent increase in vaccine cost-effectiveness studies in Canada, with more studies adhering to Canadian economic evaluation guidelines. However, the two Canadian cost-effectiveness studies looking at universal chickenpox vaccination were conducted prior to the implementation of the program in Canada, and with limited knowledge of the actual cost or effectiveness of the vaccine. We built an agent-based model (ABM) to aid in the understanding of chickenpox and shingles disease dynamics, to measure the cost-effectiveness of chickenpox vaccination and to help guide health policy decision-making. Chickenpox is a childhood disease caused by varicella-zoster virus (VZV), which can reactivate as shingles in adulthood. While natural waning of VZV cell-mediated immunity (CMI) can lead to shingles reactivation, one theory posits contact with a shingles or chickenpox case may boost an individual’s VZV-CMI (i.e. exogenous boosting), offering protection from shingles. Using the ABM, we tested several quantitative theories of VZV boosting, as well as the impact of chickenpox vaccination on shingles epidemiology. Our model highlighted the importance of not only knowing when, and if, the VZV exogenous boosting events occur but the duration an individual remains immune following a boosting event. In Canada, there are eight different schedules, including diverse types of vaccines, ages of administration, number of doses for the chickenpox vaccine. Using the ABM we were able to test the effectiveness and cost-effectiveness of two main chickenpox vaccine schedules (schedule 1- MMRV at 12 months and 4-6 years; schedule 2- MMRV at 12 months and 18 months). We found differences in effectiveness and costs between the two schedules were relatively minor, suggesting other considerations, such as the current vaccine strategy and public preference may play a bigger role in determining the most appropriate chickenpox vaccination schedule.