What are Adverse Events Following Immunisation?

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.