children, it would be difficult to assess whether those differences are due to the dozen or so antigens encountered in vaccines or to the myriad of antigens a child encounters through infections or diet.

There are several immunologic parameters that one could study, a vaccine manufacturer pointed out, and it is not known which are of the highest priority for documenting potential changes with respect to vaccination. It is not practical and there are not enough resources, he said, to evaluate all immune parameters. Ultimately, the immune parameters selected should relate to protection from the agent as well as related to adverse events. One researcher noted that vaccines have mainly been evaluated in terms of their ability to generate antibodies. She suggested conducting more studies that determine T-cell response or other immune parameters besides antibody response, as is already being done for AIDS vaccines. Even if immunologic differences were found when individuals are exposed to an antigen via a vaccine versus via natural infection, a researcher noted, those differences will be difficult to interpret in terms of their implication for susceptibility to disease or the development of subsequent disease.

The second important area of research, Fisher said, is to assess whether vaccinations in conjunction with antibiotic use and exposure to environmental toxins make people more vulnerable to new and more virulent viruses and bacteria or more prone to adverse effects from vaccines or infections. Others questioned the scientific basis for this hypotheses.

Third, she suggested that studies be conducted to investigate the possible link between vaccination and learning disabilities or attention deficit disorder. More than 2.5 million children with specific learning disabilities have been identified in the United States (U.S. Dept. of Education, 1996). She and Kinsbourne, a pediatric neurologist, noted that a vaccine that might, in rare cases, cause severe brain damage might more frequently cause milder forms of brain damage that could manifest itself as a learning disability or attention deficit disorder. This hypothesis is based on evidence that some causes of brain damage, such as low oxygen conditions during birth, not only cause severe mental retardation or cerebral palsy but are also associated with an increased risk of the hyperactive impulsive behavior that is seen in children with attention deficit disorder (Nichols and Chen, 1981; Astbury et al., 1987).

If a vaccine causes mild brain damage, the effects of such damage may not be apparent for several years following vaccination, Kinsbourne pointed out. Deficits in such skills as learning to read, spell, write, or concentrate, for example, will not become apparent until the child has reached a level of development where such skills are expected. Kinsbourne also noted that there are several different kinds of learning disabilities and that there are no established causes for any of them.

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