The most important allergy to vaccines is anaphylaxis. The diagnosis is made according to Brighton's case definition for retrospective review, but in actual clinical practice, if anaphylaxis is suspected at all, it should be treated promptly according to anaphylaxis guidelines. Although cases leading to anaphylaxis are very rare, one should always be prepared for a sudden and serious adverse reaction. Gelatin and chicken egg proteins were considered to be causes of vaccine allergy, but gelatin was removed from most vaccine products for young children and no association between vaccine anaphylaxis and egg protein-containing vaccines has been confirmed. Tests to investigate the cause include the prick test, basophil activation test, and specific IgE antibodies, but the usefulness of these tests still to be established.

Polyethylene glycol was suggested as a major culprit to cause anaphylaxis following COVID-19 vaccination, but recent evidence has denied the hypothesis, and mechanisms and the diagnostic methods for the newly emerged adverse event is still to be investigated.

The diagnostic methods for vaccine allergy must be established for each vaccine. We have to pursue safe alternatives for the patients who had anaphylaxis following vaccination to protect from the infectious disease that vaccine can only prevent.