Adverse Events Associated with Childhood Vaccines Evidence Bearing on Causality (1994) / Chapter Skim
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2 Causality and Evidence
2 Causality and Evidence
Pages 19-33
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From page 19... ...
Causality and Evidence CAUSALITY Definitions The concept of causality is of cardinal importance in health research, clinical practice, and public health policy. It also lies at the heart of this committee's charge: to make causal inferences about the relation between vaccines routinely administered to children in the United States and several specific adverse health outcomes.
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causality question for the relations between vaccines routinely administered to children and several specific adverse events. The question is conventionally approached through controlled epidemiologic studies.
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In the case of vaccines, however, dose and frequency tend to be fixed. Moreover, since some of the adverse events under consideration by the committee could represent hypersensitivity or another type of idiosyncratic reaction, the absence of a dose-response effect might not constitute strong evidence against a causal relation.
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The committee acknowledges that that which has not been reported might indeed have occurred. Instead, the committee relied on epidemiologic studies to reject a causal relation.
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But the mere fact that B follows A does not mean that A caused B; inferring causation solely on the basis of a proper temporal sequence is the logical fallacy of post hoc ergo propter hoc (liter ally, "after this, therefore because of th~s771. Many factors go into evaluating the causal relation between vaccine exposure and adverse events from individual case reports.
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causality from case reports could be applied to reports of adverse events following receipt of vaccines. The most common is global introspection (Lane, 19841.
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In evaluating the case reports available to the committee, the committee adopted an informal Bayesian approach. The main elements of the case reports used in the committee's assessments included the individual's medical history, the timing of onset of the adverse event following vaccine administration, specific characteristics of the adverse event, and follow-up information concerning its evolution.
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The Will It? causality question refers to how frequently a vaccine causes a specific adverse event and can relate to either individuals or populations.
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SOURCES OF EVIDENCE FOR CAUSALITY The sources of evidence for causality examined by the committee include demonstrated biologic plausibility, reports of individual cases or series of cases, and epidemiologic studies. In an epidemiologic study, the investigators measure one or more health-related attributes (exposures, outcomes, or both)
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From page 28... ...
Only demonstrated biologic plausibility was considered by the committee in reaching its causality judgments. Case Reports, Case Series, and Uncontrolled Observational Studies The committee obtained reports of individual cases of adverse events following receipt of vaccine through the published medical literature as well as from passive, spontaneous surveillance systems established by the vaccine manufacturers, the U.S.
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In controlled cohort studies, a defined group of individuals exposed to a given vaccine are followed longitudinally for the occurrence of one or more adverse events of interest, and the rate of such occurrence is compared with the rate in an otherwise similar group of nonexposed individuals by using either the ratio of rates (relative risk) or their difference (risk difference)
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Although randomized clinical trials are generally accepted as providing the most scientifically valid assessment of causal relations, most have been too small to contribute any useful evidence bearing on the vaccine-adverse event associations under consideration by the committee. Thus, case reports, case series, and uncontrolled observational studies and controlled observational epidemiologic studies were often the main basis for the committee's judgment.
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This is true even for the scientific "gold standard" in evaluating causal relations, the randomized clinical trial. For each vaccine-adverse event association under consideration, the committee started from a neutral position, presuming neither the presence nor the absence of a causal relation between the vaccines and the adverse events under consideration.
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No evidence bearing on a causal relation. Putative associations between vaccine and adverse events for which the committee was unable to locate any case reports or epidemiologic studies were placed in this category.
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Epidemiologic studies and/or case reports provide unequivocal evidence for a causal relation, and biologic plausibility has been demonstrated. REFERENCES Dickersin K, Berlin JA.
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