Vaccine safety

The two major aspects of any health intervention are effectiveness and safety. Side effects are known to occur with practically all medicines. Vaccine reactions are known to occur with practically all vaccines.

In the field of immunisation there are two important differences compared with other medicines. Firstly, vaccines are given to healthy individuals and, therefore, the tolerance for vaccine 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 a vaccine reaction occur infrequently, a large number may be seen.

Chance events, that are purely coincidental but could be blamed on the vaccine, are more likely to occur because of the large number of healthy individuals who receive vaccines. Reactions to vaccines may be common and expected such as injection site pain and swelling or fever (vaccine reactogenicity), uncommon and expected or rare/very rare.

Any undesirable event that occurs after getting a vaccine is called an Adverse Event Following Immunisation (AEFI) whether it was caused by the vaccine or not.