I am no PV expert but in my experience company SOPs usually prefer the safer option of over-reporting when there is no clear causality, than under-reporting, as it is treated as if no causality was provided by the Investigator but this is usually described within company SOPs. ICH PV E2A states "Many terms and scales are in use to describe the degree of causality (attributability) between a medicinal product and an event, such as certainly, definitely, probably, possibly or likely related or not related. Phrases such as "plausible relationship," "suspected causality," or "causal relationship cannot be ruled out" are also invoked to describe cause and effect. However, there is currently no standard international nomenclature. The expression "reasonable causal relationship" is meant to convey in general that there are facts (evidence) or arguments to suggest a causal relationship". The absence of commitment from the Investigator by using unlikely means a "causal relationship cannot be ruled out" (ICH PV E2A) so in my opinion (and this is just my opinion as am no PV expert) the event should be reported as related.