Vaccine anaphylaxis — DCH MCQ

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HardImmunisationVaccine anaphylaxisDCH

A 4-month-old infant is observed after routine immunisation. Four minutes later, the infant becomes pale, floppy and poorly responsive, with a persistent cough, bilateral wheeze, a heart rate of 190 beats/min and weak central pulses. There is no urticaria or angioedema, and the infant does not improve when laid flat. What is the most likely diagnosis?

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Correct answer: B — Vaccine-associated anaphylaxis

The correct answer is B, vaccine-associated anaphylaxis. The rapid onset after immunisation, bronchospasm, tachycardia, weak pulses and persistent impaired responsiveness demonstrate acute Breathing and Circulation compromise. Urticaria or angioedema is not required: skin or mucosal changes are absent in 10–20% of anaphylactic reactions. A vasovagal episode usually causes a slow or normal pulse, has no wheeze and improves rapidly when the patient lies flat. A hypotonic-hyporesponsive episode can cause pallor, floppiness and reduced responsiveness in an infant, but does not explain bronchospasm and cardiovascular compromise. A febrile seizure requires fever with seizure activity, while an expected local reaction is confined predominantly to the injection site.

Reference: UK Health Security Agency. Immunisation against infectious disease: Green Book, Chapter 8, Vaccine safety and the management of adverse events following immunisation, sections on hypotonic-hyporesponsive episodes and recognition of anaphylaxis. 28 May 2025. https://assets.publishing.service.gov.uk/media/686d5f2cfe1a249e937cbe53/Green_Book_chapter_8_28May25.pdf