Age-related processes play a very important role in the population dynamics of hepatitis B virus (HBV) (Medley et al., 2001). This infection has similarly been estimated to result in 1 million deaths each year, but mainly in adulthood as a result of liver cancer and cirrhosis; as with measles, however, there is a safe and effective vaccine. In contrast to measles, infection with HBV can result in persistent infection over a period of decades, and additionally the probability of persistent infection is much greater if infection occurs in early childhood. The modes of transmission of HBV are similar to those of HIV (though HBV is very much more infectious), such as through sexual contacts, health interventions, intravenous drug use (IVU) or at birth from mother to child (Edmunds et al., 1996; Williams et al., 1996). These processes all have a strongly age-related or age-determined component. Also, in general, the endpoint of many persistent HBV infections, such as liver cancer or cirrhosis, is not reached for several decades, so that persistent HBV infection arising in childhood (e.g., through maternal transmission) or in young adulthood (perhaps through sexual contacts or IVU) often becomes a cause of severe disease or death in middle-aged or older adults. This long time span of infection means that the population dynamics of HBV is quite different from that of measles, showing no regular oscillations and, in the absence of sharp changes in behavioral trends, prevalence of infection will change only over a period of decades. In these circumstances even a program vaccinating all susceptibles in every age with 100% efficacy will take decades before persistent infection can be eliminated, and over this extended period the reservoir of persistent infection and associated disease would be found increasingly in older age groups.
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