Distinguishing vasovagal episodes from anaphylaxis reactions following immunisation

Immunisation providers must be able to distinguish between a vasovagal episode (faint), convulsion and anaphylaxis.

A vasovagal episode (faint) is relatively common after the vaccination of adults and adolescents, but infants and children rarely faint.

Anaphylaxis following routine vaccination is very rare, but can be fatal.

Sudden loss of consciousness in young children should be presumed to be an anaphylaxis, particularly if a strong central pulse is absent. A strong central pulse (carotid) persists during a vasovagal episode or convulsion.

For a printable version, download the Vasovagal Episode or Anaphylaxis chart (PDF 261KB).

Onset differences

Vasovagal — Immediate, usually within minutes of, or during, receiving the vaccine

Anaphylaxis — Usually within 15 minutes but can occur within hours of receiving the vaccine

Respiratory differences

Vasovagal Normal respiration, may be shallow but not laboured


  • cough, wheeze, stridor, hoarseness
  • signs of respiratory distress (tachypnoea, cyanosis, rib recession)
  • upper airway swelling (lip tongue, throat, uvula or larynx)

Cardiovascular differences


  • bradycardia weak/absent peripheral pulse, but with strong central pulse (carotid)
  • hypotension, usually transient and corrects in supine position
  • loss of consciousness; improves once supine or head down position


  • tachycardia, weak/absent central pulse
  • hypotension; sustained and no improvement without specific treatment (in infants and young children, limpness and pallor are signs of hypotension)
  • loss of consciousness; no improvement once supine or in head down position

Skin differences

Vasovagal Generalised pallor, cool clammy skin


  • skin itchiness
  • generalised skin erythema (redness)
  • urticaria (weals)
  • angioderma (localised or generalised swelling of the deeper layers of the skin or subcutaneous tissues)

Gastrointestinal differences

Vasovagal Nausea, vomiting

Anaphylaxis Abdominal cramps, diarrhoea, nausea and /or vomiting

Neurological differences

Vasovagal Feels faint, light-headed

Anaphylaxis — Sense of severe anxiety and distress

Management of anaphylaxis

Rapid intramuscular (IM) administration of adrenaline is the foundation for treatment of anaphylaxis.


  • If patient unconscious, place on left side and maintain airway.
  • If patient conscious, place in supine position, elevate legs (unless results in breathing difficulties).
  • If showing signs of anaphylaxis give adrenaline by deep intramuscular injection into the anterolateral thigh. Must not be given intravenously.
  • Call for assistance. Never leave patient alone.
  • Administer oxygen if available by face mask at a high flow rate.
  • If no improvement in patient’s condition within 5 minutes, repeat doses of adrenaline every 5 minutes until condition improves or medical assistance arrives
  • Check breathing. If absent, commence basic life support or appropriate CPR as per the Australian Resuscitation Council guideline.
  • Transfer to hospital for further observation and treatment.
  • Document the event including time(s) and dose(s) of adrenaline administered.

Adrenaline dose

Adrenaline 1:1000 = 0.01 ml/kg (0.01mg/kg) deep IM injection in thigh

Less than 1 year (approx 5-10 kg) 0.05 to 0.1mL

1 to 2 years (approx 10 kg) 0.1 mL

2 to 3 years (approx 15 kg) 0.15 mL

4 to 6 years (approx 20 kg) 0.2 mL

7 to 10 years (approx 30 kg) 0.3 mL

10 to 12 years (approx 40 kg) 0.4 mL

Greater than 12 years and adults, including pregnant women (over 50kg) 0.5 mL

Use a 1 mL syringe to accurately measure the dose.