Under India's Universal Immunization Programme, the Vaccine Vial Monitor (VVM) on an oral polio vaccine vial shows that the inner square has become slightly darker than the outer circle but the two are still distinguishable. According to WHO/UIP guidelines, this VVM reading indicates:

• A VVM Stage 1 — vaccine is safe to use
• B VVM Stage 3 — discard the vial as vaccine quality is compromised
• C VVM Stage 2 — vaccine may be used but should be used first (use first, store last) ✓
• D VVM Stage 4 — vaccine has been exposed to excessive heat, do not use

Explanation

VVM has 4 stages: Stage 1 (inner square lighter than outer circle — safe to use), Stage 2 (inner square has darkened but is still lighter than outer circle — 'use first', deprioritize for storage), Stage 3 (inner square same colour as outer circle — discard point reached, do not use), Stage 4 (inner square darker than outer circle — well past discard point, do not use). The question describes Stage 2 — the vaccine should be used first (open and use before other vials) but need not be immediately discarded. The VVM monitors cumulative heat exposure, not just single temperature excursions.

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

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Written and medically reviewed by the StethoPrep medical team.