Anaphylaxis is a life-threatening allergic reaction to foreign protein antigens such as food, drugs, vaccines and bee stings. It is a very rare complication of immunisation (0.4-2 per million doses). Most episodes begin within 30 minutes of vaccination. Shorter intervals to onset generally indicate more severe reactions. However due to the unpredictable nature of anaphylaxis it is not possible to define a particular time period over which all individuals should be observed following immunisation. When possible, patients should remain in the vicinity of the place of vaccination for up to 15 minutes as typically onset of anaphylaxis occurs within minutes.
Anaphylaxis algorithm update - Immunisation Guidelines Ireland.pdf