Bloom and colleagues [1] present a careful analysis of the cost-effectiveness of alternative hepatitis B virus immunization strategies. However, the conclusion that various hepatitis B virus screening and vaccination strategies are an inexpensive way to improve the health of the public may be stronger than they imply, at least in the intermediate years of the program.

Widespread vaccination reduces not only the chance of vaccinees becoming infected but also the risk for spreading the virus to others. By decreasing the population prevalence of transient or long-term viral carriers, health benefits accrue to those who are not immunized. Contagion models developed to predict the effect of this "herd immunity" on specific population-based programs are complex and require data not generally available [2, 3]. Generally, though, as vaccination programs proceed, the risk faced by unvaccinated persons decreases, because as more become immune, there are fewer people from whom one might catch the virus.

With a very successful program, the chance of infection might in time become so low that vaccination no longer confers many benefits at all. In this circumstance, persons might reasonably forego vaccination, because to them the costs simply are not worth the benefits. In effect, these persons take a "free ride" on the herd immunity provided by the past vaccination decisions of others [4]. Experimental evidence suggests that, indeed, some persons use this strategy when deciding whether to be immunized [5].

Whether one views herd immunity as further reducing the incidence of disease or as reducing the need for a vaccine, the general point is that herd immunity can be an important consideration in models of contagious disease. The risk for hepatitis B virus infection ought to change through each 10-year cycle of the model, not only because newborns, adolescents, and adults have different risks for infection but also because of the overall decrease in the prevalence of carriers we would expect from the vaccination efforts of earlier cycles. In short, immunizing our children will protect our grandchildren.