A Vaccine Vial Monitor (VVM) on a BCG vial shows that the inner square is darker than the outer circle. The correct action is:

• A Use the vaccine; the darker square indicates cold chain was maintained
• B Reconstitute and use within 2 hours because the change is irreversible
• C Store the vial at −20°C to reverse the colour change
• D Discard the vial as the cumulative heat exposure has compromised potency ✓

Explanation

VVM Stage 3/4 (inner square colour darker than or matching the outer circle) indicates the discard point — cumulative heat exposure has likely degraded the vaccine beyond acceptable potency. The VVM inner square irreversibly darkens with heat; this change cannot be reversed by re-cooling. A lighter inner square relative to outer circle (Stage 1 or 2) means the vial is acceptable for use. Discarding is mandatory at Stage 3 or beyond.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.