AEFI: Definitions of Terms

Adverse event following immunisation (AEFI) is any untoward medical occurrence which follows immunisation and which does not necessarily have a causal relationship with the usage of the vaccine. The adverse event may be any unfavourable or unintended sign, abnormal laboratory finding, symptom or disease. These are the definitions of terms that are used in relation to adverse event following immunisation (AEFI).

Causal association: A cause-and-effect relationship between a causative (risk) factor and an outcome. Causally associated events are also temporally associated (i.e. they occur after vaccine administration), but events which are temporally associated may not necessarily be causally associated.

Causality assessment: In the context of AEFI surveillance, causality assessment is a systematic review of data about AEFI case(s) in order to determine the likelihood of a causal association between the event and the vaccine(s) received.

Cluster: Two or more cases of the same or similar events related in time, geography (place), and/or vaccine administered. AEFI clusters are usually associated with a particular supplier/ provider, health facility, and/or a vial of vaccine or a batch of vaccines.

Coincidental events: An AEFI that is caused by something other than the vaccine product, immunisation error or immunisation anxiety.

Contraindication: A situation where a particular treatment or procedure, such as vaccination with a particular vaccine, must not be administered for safety reasons. Contraindications can be permanent (absolute), such as known severe allergies to a vaccine component, or temporary (relative), such as an acute/severe febrile illness.

Injection safety: The public health practices and policies dealing with various aspects of the use of injections (including adequate supply, administration and waste disposal) so that the provider and recipient are not exposed to avoidable risks of adverse events (e.g. transmission of infective pathogens) and creation of dangerous waste is prevented. All injections, irrespective of their purpose, are covered by this term (see definition of safe injection practices).

Immunity: The ability of the human body to tolerate the presence of material “indigenous” to the human “body” (self) and to eliminate “foreign” (non-self) material. This discriminatory ability provides protection from infectious diseases, since most microbes are identified as foreign by the immune system.

Immunisation safety: The process of ensuring the safety of all aspects of immunisation, including vaccine quality, adverse events surveillance, vaccine storage and handling, vaccine administration, disposal of sharps and management of waste.

Immunisation safety surveillance: A system for ensuring immunisation safety through detecting, reporting, investigating and responding to AEFI.

Non-serious AEFI: An event that is not “serious” and does not pose a potential risk to the health of the recipient. Non-serious AEFI should also be carefully monitored because they may signal a potentially larger problem with the vaccine or immunisation, or may have an impact on the acceptability of immunisation in general.

Safe injection practice: Practices which ensure that the process of injection carries the minimum of risk, regardless of the reason for the injection or the product injected.

Serious AEFI: An event that results in death, is life-threatening, requires in-patient hospitalisation or prolongation of existing hospitalisation, results in persistent or significant disability/ incapacity, or is a congenital anomaly/birth defect. Any medical event that requires intervention to prevent one of the outcomes above may also be considered as serious.

Signal: (safety signal) Information (from one or multiple sources) which suggests a new and potentially causal association, or a new aspect of a known association, between an intervention and an adverse event or set of related adverse events, that is judged to be of sufficient likelihood to justify verificatory action.

Surveillance: The continuing, systematic collection of data that are analysed and disseminated to enable decision-making and action to protect the health of populations.

Trigger event: A medical incident following immunisation that stimulates a response (usually a case investigation).

Vaccine: A biological preparation that improves immunity to a particular disease. In addition to the antigen, it contains multiple components (excipients) and each component may have unique safety implications.

Vaccine pharmacovigilance: The science and activities relating to the detection, assessment, understanding and communication of AEFI and other vaccine- or immunisation-related issues, and to the prevention of untoward effects of the vaccine or immunisation.

Vaccine product-related reaction: An AEFI that is caused or precipitated by a vaccine due to one or more of the inherent properties of the vaccine product, whether the active component or one of the other components of the vaccine (e.g. adjuvant, preservative or stabiliser).

Vaccine quality defect-related reaction: An AEFI that is caused or precipitated by a vaccine that is due to one or more quality defects of the vaccine product, including its administration device as provided by the manufacturer.

Vaccination failure: Vaccination failure may be defined on the basis of clinical endpoints or immunological criteria where correlates or surrogate markers for disease protection exist. Primary failure (e.g. lack of seroconversion or sero-protection) needs to be distinguished from secondary failure (waning immunity). Vaccination failure can be due to (i) failure to vaccinate (i.e. an indicated vaccine was not administered appropriately for any reason) or (ii) because the vaccine did not produce its intended effect.

Vaccine reaction: An event caused or precipitated by the active component or one of the other components of the vaccine. It may also relate to a vaccine quality defect.

Vaccine safety: The process that maintains the highest efficacy of, and lowest adverse reaction to, a vaccine by addressing its production, storage and handling. Vaccine safety is a part of immunisation safety.


Definition and application of terms for vaccine pharmacovigilance. Report of the CIOMS/WHO Working Group on Vaccine Pharmacovigilance. Geneva: Council for International Organizations of Medical Sciences; 2012 ( accessed 25 July 2014).

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