Adverse events: Frequency and severity

Under recommended conditions, vaccines should cause no adverse reactions and completely prevent the infection that they target. Unfortunately, current technology does not allow for such perfection. The key, therefore, is to minimize adverse events as much as possible and ensure a safe use of vaccines.

Adverse events following immunization (AEFIs) are classified by the cause of the event. As you have learned previously, when an AEFI is caused by the properties of the vaccine, it is classified as a vaccine (product or quality related) reaction. Other categories include immunization error-related, and immunization anxiety-related reactions and coincidental events.

Frequency and severity of adverse vaccine reactions

Frequency Occurrence among persons vaccinated in percent Severity of reactions
Very common ≥ 10% Common and usually minor reactions:
  • Are part of the immune response to vaccine,
  • Reactions settle on their own,
  • Examples include:
    • Fever,
    • Malaise.
Common (frequent) ≥ 1% and < 10%
Uncommon (infrequent) ≥ 0.1% and < 1% Rare, usually more severe reactions:
  1. Usually require clinical management,
  2. Examples include:
    • Severe allergic reaction (e.g., anaphylaxis) including an exaggerated response to the vaccine antigen or component,
    • Vaccine specific reactions, such as BCG osteitis.
Rare ≥ 0.01% and < 0.1%
Very rare < 0.01%


Which of the following statements is wrong:

A. An event that occurs in 12 out of a hundred persons is regarded as very common.
B. An event that occurs in 2 out of a hundred persons is regarded as common.
C. An event that occurs in 1 out of 20,000 is regarded as very rare.
D. An event that occurs in 2 out of a thousand persons is regarded as common.
E. An event that occurs in 1 out of 9,000 is regarded as rare.


Answer D is wrong.

An event that occurs in 2 out of a thousand persons is regarded as uncommon (infrequent).

Please compare the frequency and the percentage of persons vaccinated in the table above.

Background rates

Background rates of vaccine adverse reactions worldwide are published by WHO. Background rates differ from country to country because of differences in national surveillance systems. Understanding the background rates in a specific population is useful for monitoring the sensitivity of the vaccine pharmacovigilance system in detecting changes in the frequency of vaccine reactions.

For example, using the background rate in comparison to the observed rate can be helpful to determine the reaction rate of a vaccine (see graphic).

Fever following vaccination
Example: Fever following vaccination

Any increase in the frequency of AEFIs should alert you to consider the quality of the vaccine and whether there are special risks in local populations. In addition, knowing when vaccine reactions may appear (time to onsetTime to onsetThe period of time between an intervention (in this case, a vaccination) and the onset of an adverse reaction to the vaccine.) is useful for investigating and verifying cases, as Module 4 will describe.

Key point

Knowing the background rates in your population is essential in detecting changes in the frequency of vaccine reactions and identifying trends of concern, such as rates reported by AEFI surveillance that are higher than expected.