Cartoon image of a man showing his arm where he received a vaccination

Adverse events

Any medical event occurring after immunisation can be classified as an adverse event following immunisation (AEFI) whether the event has any causal relationship to getting the immunisation or not.

Adverse events/responses following immunisation can range from mild to severe and from common to very rare. They may occur as a result of the immunisation, a causal relationship between the event and immunisation, or occur after the immunisation by chance, a coincidental relationship between the event and immunisation.

Adverse events following immunisation can be classified as:

  • Vaccine responses
    • Product-related
    • Quality defect-related
    • Error-related
  • Immunisation anxiety-related responses
  • Coincidental events

Vaccine responses

Common, mild vaccine responses

These are noticeable local or systemic symptoms occur as the immune system responds to the vaccine. Local responses include pain, redness and/or swelling around the site of injection. Systemic responses include responses such as fever, headache, feeling tired, irritable, or generally unwell.

Mild responses to vaccines usually occur soon after immunisation and resolve spontaneously within a few days without any specific treatment. Onset of mild responses to injected live vaccines could take a week or more to occur but they also resolve spontaneously without any specific treatment.

Some people feel more comfortable when they manage their symptoms, for example, placing a cool, wet cloth on a painful injection site, resting, or using mild analgesia to relieve discomfort associated with injection site responses or fever.

Rare, severe vaccine responses

These responses also occur as the immune system responds to the vaccine. They include responses such as a severe allergic reaction (anaphylaxis) to an ingredient in the vaccine, or a low platelet count (thrombocytopenia), febrile seizures, HHE (hypotonic, hyporesponsive episode), or prolonged crying.

Anaphylaxis can be treated and is therefore rarely life threatening. Other severe responses may or may not require treatment to help them resolve and rarely result in any long-term consequences.

Anxiety-related responses to immunisation

Any individual could feel tense or anxious as they wait for and finally receive a vaccine, particularly when the vaccine is administered by injection. For some individuals these feelings will be so mild they don’t notice them, but for others they could be overwhelming. Symptoms of anxiety related to the process of immunisation could include breath-holding with or without a brief loss of consciousness in children, vomiting, breathing too fast (hyperventilation) and feeling light-headed, tingling around the mouth or in the hands, fainting, or convulsing. These symptoms could occur in many other anxiety-related situations and are not specific to immunisation. They will resolve by themselves once the state of anxiety reduces. These are also known as immunisation-stress related responses (ISRR), see here for more information.

Coincidental events

These events are completely unrelated to immunisation. They occur after immunisation by chance and are likely to have occurred even if the immunisation had not been given.

Medical events occur throughout life. Some affect people at any time of their life. Others are seen more frequently in certain age groups, or simply noticed and/or diagnosed more frequently in certain age groups.

The growth and development of infants and young children is very visible and closely monitored by parents and health professionals. Most immunisations are offered in early childhood, so children can develop protection against preventable diseases. This is also the age when any congenital or developmental conditions are often noticed.

As large numbers of children are immunised at a time in their lives when congenital or developmental conditions are likely to be noticed,  it is inevitable that one or more of these events will occur after an immunisation.

This can create the appearance that illnesses or newly diagnosed conditions are related to immunisation when they are unrelated.


World Health Organization. Vaccine safety basics learning manual [Internet]. Geneva: World Health Organization; 2013 cited 2017 March 30]. Available from:

Last updated:
Oct 2022