The Casey decision by Special Master Sweeney illustrates an inference pattern that may incorporate policy considerations unique to Vaccine Act cases.
In her reasoning through Prong 1 of the Althen test of causation in fact, the special master was persuaded that the varicella vaccine can in fact cause a direct viral infection or an immune-mediated inflammatory response. The first prong of the Althen test requires that the petitioner prove, by a preponderance of the evidence, that there is “a medical theory causally connecting the vaccination and the injury.” (Althen, 418 F.3d at 1278.) The evidence in Casey in this regard can be modeled as follows (using the causal chain to a direct viral infection):
The reasoning is that if a natural varicella infection can cause a direct viral infection, and the varicella vaccine contains a live but attenuated varicella virus, then “it is reasonable to assume that the virus can multiply once inside the body” – at least in some people, if there is insufficient countervailing evidence. This inference might be influenced by the policies behind the Vaccine Injury Compensation Program.
Note the last sentence of the paragraph in which the special master provides her finding on Prong 1 of Althen:
As we find more instances of this type of default reasoning in the Vaccine Act cases, we need to try to flesh out what this policy component amounts to, and the conditions on its warranting such an inference.