Reactogenicity

In clinical trials, the term reactogenicity refers to the property of a vaccine of being able to produce common, "expected" adverse reactions, especially excessive immunological responses and associated signs and symptoms, including fever and sore arm at injection site. (Much less frequently, the term has also been applied to therapeutic drug trials.) Other manifestations of reactogenicity typically identified in such trials include bruising, redness, induration, and swelling.

The term reactogenicity was coined by the US Food and Drug Administration (FDA). Typically, reactogenicity is observed upon the administration of an adjuvant, which is a chemical additive intended for enhancing the recipient's immune response to the antigen that is present in a vaccine, but can also be observed in non-adjuvanted vaccines. Reactogenicity describes the immediate short-term reactions of a system to vaccines and should not be confused with the long-term consequences sequelae. Assessments of reactogenicity are carried out to evaluate the safety and usability of an experimental vaccine (see Investigational New Drug). It is unclear whether a higher degree of reactogenicity to a vaccine correlates with more severe adverse events, which would require hospitalization or are life-threatening. Adverse events have been linked to a higher degree of reactogenicity; however, the links might have been coincidental. After assessing large databases relating to these events for many years, the FDA has not been able to make such correlation.

Formal definitions

The US National Institutes of Health (NIH) has provided the following "definition of reactogenicity" in a suggested template for clinical trials protocols:

Reactogenicity events are adverse events that are common and known to occur for the intervention/investigational product being studied and should be collected in a standard, systematic format using a graded scale based on functional assessment or magnitude of reaction. Provide a definition of expected vs. unexpected AEs and local vs. systemic events, based on the risk profile of the intervention/investigational product. This information is found on the CIB or package insert. Typically, reactogenicity adverse events are solicited and collected on diary cards or a reactogenicity case report form.

Example of a functional scale for assessing reactogenicity or other parameters not specifically listed in the toxicity table:

0 = Absence of the indicated symptom

1 = Mild (awareness of a symptom but the symptom is easily tolerated)

2 = Moderate (discomfort enough to cause interference with usual activity)

3 = Severe (incapacitating; unable to perform usual activities; requires absenteeism or bed rest)

4 = Life-threatening