Hepatitis B virus (HBV) is one of the most common viral infections in many parts of the world. There are an estimated 300 million carriers of the virus worldwide, each of whom has a high probability of suffering chronic liver disease. There is a safe and effective vaccine against HBV which does not interfere with other vaccines commonly given in childhood and would therefore appear to be ideally suited to mass cohort immunisation. However the vaccine is expensive compared to other Expanded Programme of Immunisation vaccines and there is evidence that vaccine induced immunity declines with time. As the epidemiology of HBV is complex (see below) the outcome of mass immunisation is difficult to predict. The aim of this work is to use a mathematical model of the transmission of HBV to aid the design of vaccination programmes in developing countries.

The epidemiology of HBV has a number of interesting features which complicate the dynamics of infection. Infection with HBV can lead to long-term carriage of the virus. Furthermore the propensity for individuals to develop this chronic carrier state is related to the age at infection in a highly nonlinear manner. The probability of developing the chronic carrier state is highest amongst infants (approximately 0.9), then rapidly declines, and levels off in late childhood so that older children and adults have approximately a 1 in 10 chance of becoming carriers if infected. The epidemiological study of HBV is further complicated by its modes of transmission.