Anaphylaxis is a very rare allergic reaction (one in a million vaccinees), unexpected, and can be fatal if not dealt with adequately. Vaccine antigens and components can cause this allergic reaction. These reactions can be local or systemic and can include mild-to-severe anaphylaxis or anaphylactic-like responses (e.g.generalized urticaria or hives, wheezing, swelling of the mouth and throat, breathing difficulties, hypotnsion and shock). Reports of anaphylaxis are less common in low- and middle-income countries compared to high-income countries, probably because of reduced surveillance sensitivity and as the event may not be recognized (i.e. death attributed to another factor).
Misdiagnosis of faints and other common causes of collapse, such as anaphylaxis, can lead to inapropriate treatment (e.g.use of adrenaline and failure to recognize and treat other serious medical conditions).