What is an adverse event following immunisation?

The World Health Organization defines an adverse event following immunisation (AEFI) as ‘any untoward medical occurrence that follows immunization’. 

This means that even if the medical event wasn’t caused by the vaccine, but is coincidental to it, it is still called an AEFI.

This encourages everyone (doctors, nurses or even the public) to report anything unusual that happens after someone gets a vaccine.