Global Vaccine Safety

Causality assessment of an adverse event following immunization (AEFI)

User manual for the revised WHO AEFI causality assessment classification (Second edition)

Publication details

Number of pages: 57
Publication date: JANUARY 2018
Languages: English
ISBN: 978-92-4-151365-4
WHO reference number: WHO/HIS/EMP/SAV


In 2013 the WHO developed a revised methodology for the causality assessment of an AEFI. This new methodology incorporates a four-step process including (1) an eligibility component that reviews the diagnosis associated with the event, identifies the administered vaccines and creates a working hypothesis; (2) a checklist that systematically guides users to collate available information; (3) a decision support algorithm that assists the assessors to arrive at trends in classification; and (4) the final classification of the individual AEFI. This method was published in the “Causality assessment of an adverse event following immunization (AEFI), user manual for the revised WHO classification”.

Since its publication, there has been extensive global interest in adopting the new revised causality assessment methodology for vaccine pharmacovigilance systems. WHO provided technical support and helped build capacity in countries of all WHO Regions who wanted to use the revised methodology. A software was developed to support use of the methodology, translated to six UN languages.

The second edition of the manual was developed by the WHO with support from the Global Advisory Committee for Vaccine Safety (GACVS) based on the findings of a scientific study conducted in April 2017, “Indo- Zimbabwe inter-country study to assess the inter-rater reliability of the WHO AEFI causality assessment methodology and the utility of the new WHO AEFI causality assessment software”. The quantitative aspect of the study determined that there was realistic agreement between assessors in their findings. The qualitative aspect of the study identified areas of the methodology that could be made even more robust by the use of more accurate and clearer language, semantics and graphics. In addition recent feedback from surveillance systems and research studies have shed new evidence on areas such as, “substandard and falsified vaccines” and “immunization anxiety” that have been incorporated.