The Immunization Safety Review Committee has adopted the framework for assessing causality developed by its predecessors (convened by the IOM in 1991 and 1994 under the congressional mandate of P.L. 99–660) to address questions of immunization safety. The categories of causal conclusions used by the committee are as follows:

  1. No evidence

  2. Evidence is inadequate to accept or reject a causal relationship

  3. Evidence favors rejection of a causal relationship

  4. Evidence favors acceptance of a causal relationship

  5. Evidence establishes a causal relationship.

Assessments begin from a position of neutrality regarding the specific vaccine safety hypothesis under review. That is, there is no presumption that a specific vaccine (or vaccine component) does or does not cause the adverse event in question. The weight of the available clinical and epidemiological evidence determines whether it is possible to shift from that neutral position to a finding for causality (“the evidence favors acceptance of a causal relationship”) or away from causality (“the evidence favors rejection of a causal relationship”). The committee does not conclude that the vaccine does not cause the adverse event merely if the evidence is inadequate to support causality. Instead, it maintains a neutral position, concluding that the “evidence is inadequate to accept or reject a causal relationship.”

Although no firm rules establish the amount of evidence or the quality of the evidence required to support a specific category of causality conclusion, the committee uses standard epidemiological criteria to guide its decisions. The most definitive category is “establishes causality,” which is reserved for those relationships where the causal link is unequivocal, as with the oral polio vaccine and vaccine-associated paralytic polio or with anaphylactic reactions to vaccine administration. The next category, “favors acceptance” of a causal relationship, reflects evidence that is strong and generally convincing, although not firm enough to be described as unequivocal or established. “Favors rejection” is the strongest category in the negative direction. The category of “establishes no causal relationship” is not used because it is virtually impossible to prove the absence of a relationship with the same surety that is possible in establishing its presence.

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