The ultimate goal of an adverse event following immunisation (AEFI) investigation is to find the cause of the event and to implement follow-up actions when required. Investigation should identify any immunization error-related reactions because these are preventable. If coincidental events are recognized, proving them will be important to maintain public confidence in the immunization programme. Acknowledging that the cause of a large proportion of AEFI cannot be established by AEFI causality assessment committees due to incomplete AEFI investigation, WHO is launching an E-learning course on A