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Page 206 Afterword on Research Needs In the course of its review, the committee found many gaps and limitations in knowledge bearing directly and indirectly on the safety of vaccines. Such shortcomings relate, for example, to pathologic mechanisms of specific infectious agents, the molecular basis of vaccine injury, and the natural history of conditions such as encephalopathy, mental retardation, and chronic arthritis. Many of the reports of case series suffer from inadequate or inconsistent case definitions, variable details about cases, inclusion of nonrepresentative case groups, and failure to consider potential confounding variables or biases. In addition, existing surveillance systems of vaccine injury have limited capacity to provide persuasive evidence of causation. Many of the population-based epidemiologic studies are too small or have inadequate lengths of follow-up to have a reasonable chance of detecting true adverse effects, unless these effects are large or occur promptly and consistently after vaccination. If research capacity and accomplishment in this field are not improved, future reviews of vaccine safety will be similarly handicapped. The committee found few experimental studies published in relation to the number of epidemiologic studies published. As noted in Chapter 2, withholding of vaccines can be regarded as unethical. Although the committee was not charged with, and has not attempted, full consideration of the kinds of studies that would be both ethical and especially informative, either in the areas of vaccines that it has been charged to study or more
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Page 207 generally, it recognizes, nevertheless, that opportunities may exist for informative experiments in human populations that take advantage of the possibility of using alternative schedules for administration of vaccines. A careful review is needed to identify what sorts of questions might be best answered by further investigations and which kinds of studies could be carried out economically. The availability and introduction of new forms of pertussis vaccine, for example, could offer valuable opportunities for comparison of vaccine safety as well as efficacy. The committee is not in a position to make specific recommendations, but its experience points to fresh possibilities and to the need for such a review.
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