Adverse event, adverse effect, side effect

by Nicolas Gambardella

[Version en français]

As the deluge of communication around the covid-19 vaccines has shown us, the terminology of pharmacovigilance (the monitoring of drug safety, i.e. safety and tolerability) can lead to confusion and even feed the actors of misinformation. The World Health Organization (WHO) provides clear definitions of specific terms, unfortunately often misused.

Adverse events (événements indésirables in French) are anything that people suffer in the periods following the administration of a treatment (whether prophylactic or therapeutic). The periods involved can vary widely. One of the main tools of pharmacovigilance is the collection of reports of such adverse events. This is, for example, the role of the VAERS (Vaccine Adverse Event Reporting System) of the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) in the United States, of the ANSM (Agence nationale de sécurité du médicament et des produits de santé) in France and of the
MHRA (Medicines & Healthcare products Regulatory Agency) in the UK. The occurrence or incidence of these events is not necessarily related to the treatment. For example, in the cases of covid-19 vaccines, the MHRA listed falls, electrocutions, insect bites and car accidents. Although the incidence of these events may be affected by some drugs, this is unlikely to be the case for vaccines.

If the event is life-threatening, it is called a serious adverse event (événement indésirable grave in French). Remember the difference between severe and serious (sévère et grave in French). Severity is linked to the intensity of a phenomenon. Seriousness is related to the consequences of this phenomenon. A symptom or clinical sign can be severe without having significant implications on health and vice versa. We should note that severity depends on the personal and environmental context. Depending on the patient’s history and circumstances, an event may be mild or severe.

When the adverse event is proven to be directly related to the treatment, whether it is caused by the treatment itself or by the circumstances of its administration, it is called a treatment-emergent adverse event (événement indésirable associé aux soins in French)

An adverse effect or adverse reaction (effet indésirable in French) is an undesirable event directly caused by the treatment. Let’s note that not all adverse events of a particular type are caused by the treatment and are therefore adverse reactions. For example, thromboembolic events and myocarditis are relatively common events and are among the main complications of covid-19. Although adenovirus and mRNA vaccines, respectively, have shown an increased incidence in specific populations, further statistical analysis was required

A side effect (effet secondaire en français) is an effect that is directly caused by the treatment but is not necessarily adverse. For example, platelet aggregation inhibition by aspirin is used to prevent blood clots.

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