demyelinating disorders, specifically in health care workers and those born since 1991.

Surveillance of these outcomes would be strengthened by having standard diagnostic approaches and case definitions to permit epidemiological investigations. The lack of standardized case definition for adverse events following vaccination is a recurring concern for the committee and for all who study immunization safety. Specifically, the committee recommends the development of case definitions and guidance for diagnostic evaluation of the demyelinating disorders it has reviewed for purposes of improved vaccine adverse event surveillance and, when appropriate, causality assessment. The committee notes and encourages the work recently begun by the Brighton Collaboration to develop, through an international consensus process, a set of standard definitions for adverse events (brightoncollaboration.org); as well as the newly established Clinical Immunization Safety Assessment centers (www.cdc.gov/programs/immun8.htm). The committee has not reviewed these efforts in sufficient detail to recommend whether or not the entities behind them can or should assume the responsibility for this recommendation, or whether a distinct effort is needed.

Infection with the hepatitis B virus increases the risk of developing cirrhosis and hepatocellular carcinoma (El-Serag, 2001). Surveillance of these secondary outcomes of hepatitis B infection may provide a clearer understanding of the impact of the hepatitis B vaccination program. Continued surveillance of acute infections also remains important for the same reason. Furthermore, because hepatitis B infections can be asymptomatic, supplemental surveillance, such as the serological testing that has been conducted as part of the National Health and Nutrition Examination Survey, is needed for more accurate estimates of incidence and prevalence. Therefore, the committee recommends continued surveillance of hepatitis B disease and increased surveillance of secondary diseases, such as cirrhosis and hepatocellular carcinoma.

The committee has not recommended large-scale epidemiological studies at this time to address concern about the demyelinating conditions it has reviewed. But, in recognition of its inability to reject causality for most of these conditions, and of the limited evidence regarding biological mechanisms, the committee recommends continued research in animal and in vitro models, as well as in humans, on the mechanisms of immune-mediated neurologic disease possibly associated with exposure to vaccines.