Information for health-care workers - managing adverse events
Causal association or coincidental event ?
Immunization can be followed by adverse events from the inherent properties of the vaccine (vaccine reaction), or an error in the immunization process (programme error). The event may be unrelated to the immunization, but have a temporal association (coincidental event). Anxiety-related reactions can arise from the fear or pain of the injection rather than the vaccine. In some cases the cause of the AEFI remains unknown.
It is very difficult to be sure of the exact frequency of true adverse reactions which occur. Vaccines are mostly administered to the age group of infants and young children who are in the period of their lives when they are experiencing many illnesses. There are more health risks for the young child than for older persons. So many events which occur just after vaccination may well have occurred whether or not the child had been vaccinated. These events are "coincidental". When two sets of events can both be expected to occur frequently, it may be difficult to determine whether they are causally linked. For example, there are nine days in the life of a child which are within three days following administration of a dose of DTP (three doses times three days = nine days). In the first 180 days of a child's life, there is a one in twenty chance (nine in 180) that any event is within three days of a dose of DTP. Nonetheless, it is often very difficult to explain this idea of coincidence to parents who see their child vaccinated one day and become ill the next.
Classification of AEFI
- Vaccine reaction: event caused or precipitated by the vaccine when given correctly; caused by inherent properties of the vaccine.
- Programme error: event caused by an error in vaccine preparation, handling, or administration.
- Coincidental event: event that happens after immunization but is not caused by the vaccine - a chance association.
- Injection reaction: event from anxiety about, or pain from, the injection itself rather than the vaccine.
- Unknown: whose cause cannot be determined.