Evaluating the cost-effectiveness of vaccination programmes: A dynamic perspective

Although there are many models which are used to calculate the health benef its (and thus the cost-effectiveness) of vaccination programmes, they can b e divided into two groups: those which assume a constant force of infection , that is a constant per-susceptible rate of infection; and those which ass ume that the force of infection (at time t) is a function of the number of infectious individuals in the population at that time (dynamic models). In constant force of infection models the per-susceptible rate of infection is not altered, whereas in dynamic models mass immunization results in fewer infectious individuals in the community and thus a lower force of infection acting on those who were not immunized. We take an example of each of thes e types of model examine their underlying assumptions and compare their pre dictions of the cost-effectiveness of a mass immunization programme against a hypothetical close contact infection, such as measles. We show that if c ases of infection are the outcome of interest then the constant force of in fection model will always underestimate the cost-effectiveness of the immun ization programme except at the extremes when no one or everyone is immuniz ed. However, unlike the constant force of infection model, the dynamic mode l predicts an increase in the average age at infection after immunization w hich could impact on the estimate of the cost-effectiveness of the programm e if the risk of developing serious disease is a function of the age at inf ection (as, for instance, is the case for congenital rubella syndrome). Tak ing cases of infection as the outcome measure and using the dynamic model, the undiscounted cost-effectiveness ratio will tend to decline over time an d approach a constant value, as the system moves from pre- to post-immuniza tion equilibrium. We go on to show how the cost-effectiveness of a fixed-te rm immunization programme might change over time, and discuss why, under mo st circumstances, decision makers should not assume that elimination (permi tting termination of mass immunization) will occur. Copyright (C) 1999 John Wiley & Sons, Ltd.