The decision to introduce a new vaccine into the immunization schedule is a complex, multidisciplinary process based on assessing the risk-benefit balance and, increasingly, on estimating the cost-effectiveness of vaccination. This decision most often requires mathematical modeling to anticipate the indirect effects on the epidemiology of the target disease resulting from large-scale vaccination, effects that may be detrimental. Adding an economic component to these models ensures that the vaccination program represents an efficient allocation of available financial resources, within an increasingly constrained context. (R.A.)