Rubella Vaccines (Institute of Medicine, 1991). The pertussis and rubella vaccine committee summarized the evidence bearing on those vaccines using the following five categories: (1) no evidence bearing on a causal relation, (2) evidence insufficient to indicate a causal relation, (3) evidence does not indicate a causal relation, (4) evidence is consistent with a causal relation, and (5) evidence indicates a causal relation. They then assigned each vaccine-adverse event association under their consideration to one of these five categories.

Because some confusion has arisen over the meaning of the category descriptions used by the pertussis and rubella vaccine committee, despite extensive explanation both in footnotes and the text, the Vaccine Safety Committee adopted some minor modifications in wording intended to help in the interpretation of the present report. To facilitate reading by those familiar with the report of the previous committee, the present committee maintained both the number of categories (five) and the order of those categories but modified the wording in an attempt to clarify its meaning.

The names and descriptions of the categories used in this report are as follows:

1. 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. Demonstrated biologic plausibility alone was considered insufficient to remove a given vaccine-adverse event association from this category.

2. The evidence is inadequate to accept or reject a causal relation.

One or more (in some instances there were many) case reports or epidemiologic studies were located by the committee, but the evidence for a causal relation neither outweighed nor was outweighed by the evidence against a causal relation. The presence or absence of demonstrated biologic plausibility was considered insufficient to shift this balance in either direction.

3. The evidence favors rejection of a causal relation.

Only evidence from epidemiologic studies was considered as a basis for possible rejection of a causal relation. Such evidence was judged as favoring rejection only when a rigorously performed epidemiologic study (or a meta-analysis of several such studies) of adequate size (i.e., statistical power) did not detect a significant association between the vaccine and the adverse event. The absence of demonstrated biologic plausibility was considered supportive of a decision to reject a causal relation but insufficient on its own to shift the balance of evidence from other sources.

4. The evidence favors acceptance of a causal relation.

The balance of evidence from one or more case reports or epidemiologic



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