A health worker inspects a vial of oral polio vaccine and notes its vaccine vial monitor (VVM): the inner white square has become slightly darker than the outer blue circle. What does this indicate?
- A The vial is potent and safe to use
- B The vial must be discarded because cumulative heat exposure has exceeded the limit ✓
- C The vial has been frozen and must be discarded
- D The reading is irrelevant for polio vaccine because OPV carries no VVM
Correct answer: B. The vial must be discarded because cumulative heat exposure has exceeded the limit
Explanation
On a VVM, the inner square darkens irreversibly with cumulative heat exposure. When the inner square matches the outer circle colour, the end point is reached and the vial must be discarded even if expiry date has not passed. The VVM detects heat damage, not freezing; freeze damage requires the shake test. OPV vials do carry VVMs precisely because heat sensitivity makes field-level monitoring essential during outreach sessions.
Reference: WHO Manual on the Management of Cold Chain and Immunization Operations, 2015 ed.
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Written and medically reviewed by the StethoPrep medical team.