Post-Authorization Monitoring of Vaccine Safety

By: Noémi Bulik, Medical Writer

When it comes to reducing the burden of some infectious diseases with high morbidity and mortality, vaccination is one of the most cost-effective public health interventions.

Licensed vaccines are generally considered safe; however, adverse events may sometimes occur following vaccination. Considering that vaccines are administered to the public from a young age, vaccine safety is one of the key factors that influence confidence in vaccination programs, especially in an era in which distrust in vaccines has an increasing trend in some communities.

An adverse event following immunization represents any untoward medical event that may appear after vaccine administration, but that does not necessarily have a causal relationship with the vaccine. Although most adverse events are mild and transient, serious adverse events can sometimes occur, however, with very low frequency. Any potential safety concern can delay acceptance or even lead to refusal of vaccines at the individual level, a behavior known as vaccine hesitancy. Other factors that may contribute to vaccine hesitancy are biased or incomplete information sources, perceptions of disease risk and lack of trust in healthcare systems.

Therefore, timely and appropriate detection of safety signal and investigation of potential adverse events following vaccination need to be ensured, as well as proper communication to the general public. There are some factors which should be considered when evaluating a safety signal potentially related to vaccination, such as the background rate of the event in the target population, as coincidental adverse events may occur, vaccine administration-related errors, excipients, adjuvants and antigen types of vaccines.

The holder of the marketing authorization and national regulatory authorities continue to monitor vaccine safety after authorization. This monitoring is important since the licensed vaccines are administered to the general population and subpopulations with specific health conditions under real-world circumstances. Monitoring allows detection of unknown, rare or delayed adverse events, which may have not been detected during the pre-licensure clinical trials. Any potential adverse event is investigated, its causality with the vaccine is assessed, and the decision of causality is communicated. Identification of a vaccine-related adverse event could trigger changes in recommendations of vaccine administration in a certain subpopulation, amendments in product information or further investigation of the event through safety studies.

Post-authorization monitoring of vaccine safety is performed through spontaneous reporting systems (i.e. reports of adverse events from healthcare professionals and patients) and active surveillance systems (i.e. phase IV clinical trials, observational studies, large, interlinked databases and clinical centers such as the Clinical Immunization Safety Assessment [CISA] network). Whether or not there is a causal relationship between an adverse event and the vaccine, effective information about the benefits-risk balance of the vaccine to the public must be provided. Continuous efforts are made to ensure access for healthcare professionals to evidence-based information, to address vaccine safety concerns and to foster vaccine confidence.


Medical writers at Modis Life Sciences have expertise in developing medical communications that provide key data to the medical community through manuscripts, congress materials (abstracts, posters and oral presentations), graphical abstracts, etc. They also develop clinical writing materials, such as protocols, reports and other regulatory documents that are key throughout the life cycle of a pharmaceutical product or medical device.

– Noémi Bulik, Medical Writer




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MacDonald, N.E. and Sage Working Group on Vaccine Hesitancy, Vaccine hesitancy: Definition, scope and determinants. Vaccine, 2015. 33(34): p. 4161-4164.
Salmon DA, Dudley MZ, Glanz JM, Omer SB. Vaccine hesitancy: Causes, consequences, and a call to action. Vaccine. 2015 Nov 27;33 Suppl 4:D66-71.