A 2-year-old child develops invasive Hib meningitis despite two prior 'Hib' doses given at 6 and 10 weeks. Investigation reveals the vaccine was stored at -5°C (below 0°C freeze) before administration. The most likely cause of vaccine failure is:

• A Freezing destroys the aluminium adjuvant-antigen complex in conjugate vaccines, abrogating immunogenicity ✓
• B Vaccine interval was too short, preventing adequate memory
• C Hib conjugate vaccine is a live vaccine and freezing kills the attenuated bacteria
• D The child had pre-existing immunodeficiency preventing all vaccine responses

Explanation

Hib (Haemophilus influenzae type b) vaccine is a conjugate vaccine (polysaccharide-protein conjugate adsorbed to aluminium salt adjuvant); freezing causes irreversible aggregation of the aluminium adjuvant-antigen complex (unlike live vaccines that fail due to heat), markedly reducing immunogenicity. The shake test identifies freeze-damaged adsorbed vaccines by their failure to produce uniform suspension on shaking. This is a classic cold-chain error — DPT/Hib/Hepatitis B adsorbed vaccines must be stored at +2°C to +8°C and must never freeze.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.