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Table. Clinical features that may help differentiate between a vasovagal episode and anaphylaxis

Clinical feature	Vasovagal episode	Anaphylaxis
Onset	Immediate, usually within minutes of, or during, vaccine administration	Usually within 15 minutes of vaccine administration, but can occur within hours
Respiratory symptoms or signs	Normal breathing; may be shallow, but not laboured	Cough Wheeze Hoarseness Stridor Signs of respiratory distress, such as abnormally rapid breathing (tachypnoea), cyanosis or rib recession Upper airway swelling (eg lip, tongue, throat, uvula, larynx)
Cardiovascular symptoms or signs	Bradycardia Weak/absent peripheral pulse Strong carotid pulse Hypotension — usually transient and corrects in supine position Loss of consciousness — improves once in a lateral recumbent position	Tachycardia Weak/absent carotid pulse Hypotension — sustained and no improvement without specific treatment (Note: In infants and young children, limpness and pallor are signs of hypotension) Loss of consciousness — no improvement once in a lateral recumbent position
Skin symptoms or signs	Generalised pallor Cool, clammy skin	Pruritus (skin itchiness) Generalised skin erythema (redness) Urticaria (weals) Angioedema (localised or general swelling of the deeper layers of the skin or subcutaneous tissues)
Gastrointestinal symptoms or signs	Nausea or vomiting	Abdominal cramps Diarrhoea Nausea or vomiting
Neurologic symptoms or signs	Person feels faint or light-headed	Person has a sense of severe anxiety and distress
Note: Anaphylaxis features are modified from The Brighton Collaboration Case Definition Criteria for Anaphylaxis.⁶ Neurologic symptoms are not listed in this case definition.⁶ However, symptoms of anxiety and distress, including feelings of impending doom, are reported in people experiencing anaphylaxis.⁹

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Last updated

30 June 2023

Last reviewed

30 June 2023

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