Vaccine related anaphylaxis is a severe, sometimes life-threatening allergic reaction in which the body release histamines and other chemicals to defend against an allergen.  Initial exposure to the allergen causes the immune system to become sensitized to it.  Further exposure, subsequent to sensitization, can lead to life threatening symptoms that require immediate medical intervention

Signs and symptoms of anaphylaxis include hives or itchiness, shortness of breath, and dizziness, along with swelling of the face, tongue or throat.  Symptoms occur quickly and if not treated immediately can be life threatening.

Anaphylactic reactions have been reported after vaccine administration.