The two major aspects of any health intervention are its effectiveness and safety. Side effects, or adverse reactions, are known to occur with practically all medications.
In the field of immunisation there are two important differences compared with other drugs. Firstly, vaccines are given to healthy individuals and, therefore, the tolerance for adverse reactions is much lower. Secondly, vaccines are generally given to a large number of individuals, close to the birth cohort each year for childhood vaccines. This means that, even if adverse reactions occur infrequently, a large number may be seen. Chance events, that are purely coincidental but blamed on the vaccine, are more likely to occur because of the large number of healthy individuals who are exposed to vaccines.
Reactions to vaccines can be broadly separated into common, expected reactions such as injection site pain and swelling or fever (vaccine reactogenicity) and uncommon (occasionally serious) events which are called Adverse Events Following Immunisation (AEFI).