general use to attempt to detect groups that might be at greater risk of experiencing adverse events. Others commented that this would be difficult and costly, however, because of the increased sample sizes required. Currently, because of the need for close monitoring, accessibility to health care, and data analysis, as well as exclusion criteria for the study of experimental vaccines, clinical trials of vaccines are usually done with a homogeneous population of healthy individuals least likely to experience adverse reactions to the vaccines. Because 2 subpopulations can differ both in their risk of experiencing adverse reactions to a vaccine and in their risk of the disease that the vaccine is designed to prevent, studies to evaluate risks in specific subpopulations could provide needed information.

A consumer representative suggested that VAERS data be used to identify categories of children at higher risk of experiencing adverse events following vaccination. Once identified, these children could be screened out of vaccination programs. She also suggested using the database of the federal Vaccine Injury Compensation Program to detect common denominators among individuals who were injured or who died following vaccination. Analysis of that information could be useful for researchers to identify high-risk children who should not be vaccinated in the future. A CDC representative suggested that the large linked databases are best suited to identifying those at risk of adverse events following vaccination, once the association has been established. This individual believed that VAERS does not have the data necessary for a complete epidemiologic study to assess risk factors (i.e., information on vaccinated children who did not experience the event and on children who experienced the event but who were not vaccinated). The use of Vaccine Injury Compensation Program data would be too expensive if laboratory data were required and too imprecise if history alone were used.

An additional question that was addressed is the extent to which vaccines can trigger disorders through an immune reaction. It is known, for example, that some vaccines can trigger the Guillain-Barré syndrome, which is immune mediated. There is thus justification in studying other immune disorders to evaluate whether they could be related to vaccines. A full assessment of the safety of vaccines requires long-term studies that include controls at low risk for developing the disease prevented by the vaccine being tested, a physician suggested.

A final important question is whether there are long-latency adverse events following vaccinations. A vaccine researcher has studied several adults who have experienced deteriorating brain function over a period of years that in some cases has left them permanently comatose (Martin et al., 1994, 1995). Tests of their brain tissues, blood, and the fluid circulating in their brains and spinal cords have ruled out known causes of brain deterioration. According to



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