Purpose: A frequently used approach for assessing the health and economic impact of vaccination programs is the static model where externalities resulting from interactions between members of a community are ignored. The objective of this study was to characterize the difference between the estimates of the incremental cost-effectiveness ratios (ICER) of vaccination programs under the static approach and the dynamic approach (where externalities are incorporated) for diseases where transmission occurs.
Method: We used a general SIRS (susceptible-infected-removed-susceptible) model featuring a vaccine with several properties to illustrate the differences between the two approaches using both analytic and numerical techniques. Two special cases of this model were studied analytically focusing mainly on the behavior of the models in the steady state. We then used several sets of realistic parameters values to numerically simulate the general model and trace the transient dynamics leading to the steady state. Finally, we conducted a probabilistic sensitivity analysis (PSA).
Result: All clinical and economic measures of vaccine effects differed across the static and dynamic models. Also, the steady-state ICER of the static model is always (barring some unrealistic scenarios) higher than that of the dynamic model. The difference between the ICERs widens with increases in vaccine cost, waning immunity, quality of life, and reproduction number (Figure 1). The ratio of indirect to direct effects of a vaccine that does not fully protect against infection is always higher than that of a vaccine that confers full protection. This ratio ranges from a small to a large value, implying that the indirect effects can be even greater than the direct effects. We found that the steady-state analysis fails to capture important dynamics that may have profound effects on the results. For example, the ICER under the dynamic model may rise and then fall as the analytic time horizon expands. In all the PSA scenarios, the ICER of the dynamic model is lower than the ICER of the static model and the number of influential parameters is always higher under the dynamic model.
Conclusion: Use of a static model to evaluate the impact of vaccines can result in inaccurate predictions, and may lead to wrong conclusions. We recommend use of a dynamic model when the vaccine is likely to have important effects on transmission. 