Table 1.

WHO causality assessment criteria^a compared with CISA investigator modified criteria^b used in this report

	CISA Modified Criteria		Original WHO criteria
Definite	The report documents that the vaccine was given before the onset of the signs and symptoms and that the timing of onset was consistent with a known mechanism or published literature; there is substantial existing evidence in the medical literature establishing a causal relationship between vaccine(s) and the event, and other known causes of the event had been excluded.	Very Likely / Certain	Clinical event with plausible time relationship to vaccine administration, and which cannot be explained by concurrent disease or other drugs or chemicals
Probable	The report documents that the vaccine was given before the onset of symptoms and that the temporal relationship was consistent with a biologic mechanism or published literature; there is some evidence in the medical literature for a causal relationship between vaccine(s) and the event, and other known causes of the event had been excluded or were unlikely.	Probable	Clinical event with a reasonable time relationship to vaccine administration, and is unlikely to be attributed to concurrent disease or other drugs or chemicals
Possible	The report documents that the vaccine was given before the onset of symptoms; the medical literature does not establish or refute a causal relationship between vaccine(s) and the event, and known causes that are more likely associated with event had been excluded*.	Possible	Clinical event with a reasonable time relationship to vaccine administration, but which could also be explained by concurrent disease or other drugs or chemicals
Unlikely	The report documents that the vaccine was given before the onset of symptoms; the medical literature does not establish or refute a causal relationship between vaccine(s) and the event, and there were other known causes of the clinical event that were more likely and/or had not been excluded*.	Unlikely	Clinical event whose time relationship to vaccine administration makes a causal connection improbable, but which could plausibly be explained by underlying disease or other drugs or chemicals
Unrelated	The onset of the event was prior to vaccine administration; or there is substantial evidence in the medical literature that the vaccine does not cause the event; or there was a co-existing disease/condition, drug, or vaccine that caused the event; or the temporal relationship between vaccination and the event was not consistent with the biological onset of clinical event.	Unrelated	Clinical event with an incompatible time relationship to vaccine administration, and which could be explained by underlying disease or other drugs or chemicals.

^aCollet JP, MacDonald N, Cashman N, et al. Monitoring signals for vaccine safety: the assessment of individual adverse event reports by an expert advisory committee. Advisory Committee on Causality Assessment. Bull World Health Organ. 2000;78(2):178-185.

^bRosenberg M, Sparks R, McMahon A, Iskander J, Campbell JD, Edwards KM. Serious adverse events rarely reported after trivalent inactivated influenza vaccine (TIV) in children 6-23 months of age. Vaccine 2009 Jul 9;27(32):4278-83.