Certain findings can help establish a causation between vaccination and an adverse event. The goal is to separate an adverse event related by the vaccination from one that occurred coincidentally from another mechanism.
A general requirement is that the exact chronology of the vaccination and the adverse event is known.
Positive evidence for causation:
(1) The adverse event is one that has been previously associated with the vaccine and is accepted as a complication.
(2) The adverse event is a specific clinical syndrome (such as anaphylaxis) whose association with vaccination has a strong biologic plausibility.
(3) There is laboratory evidence confirming the association (isolation of an organism that is identical with the vaccination strain, immune response, etc)
(4) The event can be caused by readministration of the vaccine (positive rechallenge).
where:
• Positive rechallenge can be hazardous and often is not performed.
Sometimes the association between the vaccination and the adverse event may not be readily apparent or well-established, but can be demonstrated in a controlled clinical trial or epidemiologic study if there is evidence of greater risk for a specific adverse event among vaccinated vs an unvaccinated control group. This method if the study is properly designed and interpreted without bias.
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