The Casey decision by Special Master Sweeney illustrates reasoning from a “signature effect,” within prong 2 of the Althen test for causation in fact.
Prong 2 of the Althen test is that the petitioner must prove, by a preponderance of the evidence, that there is “a logical sequence of cause and effect showing that the vaccination was the reason for the injury.” (Althen, 418 F.3d at 1278.) This requirement is part of proving, not only that the vaccine can cause such an injury (“general causation”), but also that the vaccine did in fact cause the specific injury in this particular case (“specific causation”).
One segment within the Special Master’s reasoning can be modeled as follows:
This model displays what might be called reasoning from a “signature effect.” This terminology follows the lead of epidemiology, in which a “signature disease” is one that does not occur without exposure to a given toxic agent, such as asbestosis being a signature disease for asbestos. See, e.g., Federal Judicial Center, Reference Manual on Scientific Evidence, p. 381 note 128 (2d ed. 2000); Hall v. Baxter Healthcare Corp., 947 F.Supp. 1387, 1402 n. 33 (D.Or. 1996).
The petitioner’s expert, Dr. Tornatore, testified that “cerebellitis is a peculiar and specific characteristic of a natural varicella infection, and is not typically seen with other viruses,” and cited a medical publication as indicating that “because ataxia, encephalitis, and certain other symptoms characteristic of a natural varicella infection were also seen after the vaccination with the attenuated varicella virus, it was plausible that the varicella vaccine caused those symptoms.” (Casey decision at 14.) The special master summed up Dr. Tornatore’s reasoning this way: “As a result, Dr. Tornatore found petitioner’s ataxic dysarthria to be strong evidence of a varicella infection, and thus strong evidence of the varicella vaccine causing petitioner’s encephalomyeloneuritis.” Id.
The special master was persuaded by this reasoning from signature effect to cause, as seen in the following excerpt from p. 27 of the decision:
The special master had previously concluded, under prong 1 of Althen, that the varicella vaccine could probably cause the type of adverse nervous system effects exhibited by the petitioner (general causation). This reasoning probably also corroborated the medical possibility that varicella vaccine could cause cerebellitis (in general). Also, there apparently was no evidence in the record of the petitioner’s having suffered a natural varicella infection. Thus, given the special master’s finding that the petitioner had developed cerebellitis (decision at 27), the cerebellitis itself was sufficiently persuasive evidence that the petitioner’s varicella vaccination had probably caused the cerebellitis.
This reasoning from a “signature effect” to the occurrence of a particular cause is an example of reasoning “backward” along a causal chain, from effect to cause. Deductively, it is a fallacy to reason from “if A occurs, then B occurs” and “B occurs” to “A occurs,” but in the case of causation, under certain conditions, such an inference may be plausible. This is one pattern of default reasoning that we expect may play an important role in many decisions.