OPV (oral polio vaccine) is stored at −20°C in the cold chain. A technician discovers the vaccine vial temperature indicator (VVM) has turned dark (inner square darker than outer circle). What action should be taken?

• A Discard the vial; a dark VVM inner square indicates cumulative heat exposure has degraded the vaccine below acceptable potency ✓
• B VVM darkening indicates improved potency due to increased concentration; proceed with vaccination
• C Shake the vial vigorously and observe for turbidity; if clear, the vaccine is still usable
• D The VVM only monitors freezing damage; darkening is irrelevant; check for ice crystals to assess freeze damage

Explanation

The Vaccine Vial Monitor (VVM) is a time-temperature indicator attached to vaccine vials. It uses a heat-sensitive polymer that irreversibly darkens with cumulative heat exposure. In a usable vial, the inner square is LIGHTER than the outer circle. When the inner square matches or becomes DARKER than the outer circle, it indicates the vial has been exposed to unacceptable cumulative heat, degrading vaccine potency. Such a vial must be discarded and NOT administered, as the vaccine may be ineffective. The VVM does NOT monitor freeze damage — freeze indicators (FreezeTag, freeze-dried appearance) are separate. OPV is heat-sensitive (less stable than most vaccines) and freeze-tolerant.

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

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Written and medically reviewed by the StethoPrep medical team.