The unique characteristic of vaccination is that it not only reduces the incidence of disease in those immunized but also indirectly protects nonvaccinated susceptibles against infection (produces herd-immunity). The bulk of economic evaluations of vaccination programs continue to use models that cannot take into account the indirect effects produced by herd-immunity. Here, the authors illustrate the importance of incorporating herd-immunity externalities when assessing the cost-effectiveness of vaccination progams. To do this, they compare 2 methods of estimating the benefits of routine mass vaccination: one that includes herd-immunity (dynamic approach) and one that does not (static approach). Finally, they use the results to clarify a number of misconceptions that are common in the literature concerning herd-immunity and dynamical effects produced by models.