Adverse events following immunization: assessing probability of causation

The Monitoring System for Adverse Events Following Immunization of the Centers for Disease Control collects data on events temporally related to immunization. Occasionally, reports are received of neurologic disturbances temporally related to receipt of vaccine. Most of these disturbances are events that regularly occur in the absence of immunization. It is then difficult to determine whether the relationship between the immunization and illness is causal or coincidental. We developed a method to assess causation of serious neurologic events by probability theory. By combining epidemiologic information on disease incidence with specific elements of the patient history, an estimate of the odds of vaccine causation can be derived, based on rational assumptions rather than observer bias. The result is not a diagnosis but an estimate of probability.