Anaphylaxis is an acute hypersensitivity reaction with multi-organ system involvement that can rapidly progress to a severe life-threatening reaction. It has been difficult to provide a robust clinical definition of anaphylaxis because of the non-specificity of symptoms and variability of presentation. Anaphylaxis can occur to a variety of allergens, and is a rare, but well recognised adverse event following immunisation (AEFI). Several groups have recently tried to provide a working definition of anaphylaxis. This editorial reviews these definitions and discusses the challenges for vaccine safety in reliably identifying anaphylaxis as an AEFI.

Anaphylaxis may occur following exposure to allergens from a variety of sources including food, aeroallergens, venom, drugs, and immunisations. Vaccines are a mixture of compounds and allergic sensitisation can occur to any component. Individuals may be sensitised to the vaccine antigens, adjuvants (e.g., alum), excipients used in the manufacturing process (e.g., gelatin, neomycin) or a latex stopper on the vial.^1,2

Anaphylaxis as an AEFI is a concern to many health care professionals involved in the administration of immunisation programmes. An example of the level of concern can be seen by the large number of children with egg allergy referred for measles, mumps and rubella (MMR) immunisation in hospital, based on concerns about anaphylaxis. Children who have had an allergic reaction to egg, but not anaphylaxis, can be immunised without any special precautions. However, anaphylaxis is a well recognised AEFI, which may in principle occur following any immunisation without prior warning.¹ The potential for vaccines to cause anaphylaxis has two important consequences. Firstly, immunisers must be able to recognise and treat anaphylaxis in the clinic setting. Secondly, immunisation programmes must be able to reliably identify cases and to examine the potential …