Adverse events following immunizations: fact and fiction

As a public health measure, vaccines have been the most successful strategy in reducing morbidity and mortality internationally. Like all medicines, vaccines can have side effects, or adverse events following immunization (AEFI). As vaccines are predominantly administered to healthy individuals at a population level, there is an added responsibility to monitor AEFI closely. The phrase ‘primum non nocere’ or ‘first do no harm’ is ingrained in medical teaching and can be applied to vaccines, including the requirement to undergo vigorous testing in well-conducted scientific experiments before being licensed for use. Due to the inevitable limitations of sample size, common AEFI are those usually characterized in the selected populations involved in the clinical trials. Once vaccines are utilized at a population level (in different populations to those in which the clinical trials were performed), it is crucial to have systems in place to detect rare or unexpected AEFI. Capturing these events systematically to detect potential safety signals is one of the challenges for international vaccine safety. Once AEFI are reported it is important to determine whether the temporal association with the vaccine is one of causation (fact) or fiction (coincidence).