Surveillance of hepatitis B: an example of a vaccine preventable disease.

The objectives of surveillance for vaccine preventable disease vary with the stage of the vaccination programme. Pre-implementation data is required to estimate the burden of disease and to decide on the appropriate vaccination strategy. Post-implementation data is required to monitor effectiveness but when high coverage is achieved surveillance must be able to accurately identify remaining pockets of susceptible persons. Sources of data include clinical and laboratory reporting. In most countries, all vaccine preventable diseases (including acute viral hepatitis) are notifiable by law. Such systems are prone to under-reporting but are usually satisfactory for monitoring trends. To encourage the rapid tracing and vaccination of contacts of acute hepatitis B, a sensitive case definition and timely reporting system are required. A clinical definition (e.g. for viral hepatitis) may be too broad, however, to assess the impact of vaccination and additional laboratory criteria may be necessary. As a country nears elimination, the predictive value of any case definition will fall and laboratory confirmation will always be required to target policy appropriately. Serological surveillance is another method for estimating disease incidence. This may be useful for hepatitis B as tests can distinguish vaccine induced immunity from natural infection and acute from prevalent cases. To monitor vaccine impact, age-coded specimens can be collected on an intermittent basis. Where the incidence is low, however, this approach, will be very expensive. Surveillance of vaccine preventable disease therefore requires flexible surveillance systems which are able to adapt to changes in incidence of infection and in control policy. The use of multiple data sources and supportive information from special studies is essential for the valid interpretation of routine data.