The Full Form of AEFI is‍ An adverse event following immunization.

An Adverse event following immunization (AEFI) is any untoward medical occurrence which follows
immunization and which does not necessarily have a causal relationship with the usage of the vaccine.
The adverse event may be any unfavourable or unintended sign, abnormal laboratory finding, symptom
or disease

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