Table 1. Korean and WHO causality assessment criteria for adverse events following immunization.

Korean criteria		Previous WHO criteria
Definite	There is definite evidence of inoculation with the vaccine; temporal proximity in which an adverse event appeared; causal relationship between the event and vaccination rather than other causes is accepted; and the adverse event is the known reaction to the vaccine.	Very likely, certain	Clinical event with a plausible time relationship to vaccine administration, and which cannot be explained by concurrent disease or other drugs or chemicals.
Probable	There is definite evidence of inoculation with the vaccine; temporal proximity in which an adverse event appeared; and causal relationship between the event and vaccination rather than other causes is accepted.	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	There is definite evidence of inoculation with the vaccine; temporal proximity in which an adverse event appeared; it is recognized at the same level of probability that the reaction may be due to vaccination or other reasons.	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	There is definite evidence of inoculation with the vaccine; temporal proximity is not accepted when adverse events appeared, if the causal relationship between the event and vaccine is unclear.	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.
Definitely not related	There is absence of evidence of vaccine inoculation; or absence of proximity of the temporal sequence in which adverse events appeared; or presence of obvious causes resulted in the 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.

WHO, World Health Organization.