This code system http://hl7.org/fhir/reaction-event-certainty defines the following codes:

CodeDisplayDefinition
unlikelyUnlikelyThere is a low level of clinical certainty that the reaction was caused by the identified substance.
likelyLikelyThere is a high level of clinical certainty that the reaction was caused by the identified substance.
confirmedConfirmedThere is a very high level of clinical certainty that the reaction was due to the identified substance, which may include clinical evidence by testing or rechallenge.
unknownUnknownThe clinical certainty that the reaction was caused by the identified substance is unknown. It is an explicit assertion that certainty is not known.