The Swine Flu Vaccine and Guillain-Barre Syndrome: A Case Study in Relative Risk and Specific Causation

Abstract

Epidemiologic methods were developed to prove general causation: identifying exposures that increase the risk of particular diseases. Courts often are more interested in specific causation: on balance of probabilities, was the plaintiff's disease caused by exposure to the agent in question? Some authorities have suggested that a relative risk greater than 2.0 meets the standard of proof for specific causation. Such a definite criterion is appealing, but there are difficulties. Bias and confounding are well-known problems; individual differences must be considered too. The issues are explored in the context of the swine flu vaccine and Guillain-Barre syndrome. The conclusion: there is a considerable gap between relative risks and proof of specific causation.