an adverse event following a vaccination can be any health problem that occurs after receipt of the vaccination. it doesn't necessarily have to be caused by the vaccine. often such events are just coincidence and not caused by the vaccine. if it is truly caused by the vaccine it is called a reaction or side effect. the most common reactions that can be caused by vaccines are mild and resolve on their own. these include pain and redness at the injection site and fever.
we hear about adverse events [with vaccines]; is that the same as a dangerous reaction?