6.3.1 Vaccine vial monitors
A vaccine vial monitor (VVM) is a label that changes colour when the vaccine vial or ampoule has been exposed to temperatures above 8ºC over a period of time. Before opening a vaccine container, the status of the VVM must be checked to see whether the vaccine has been damaged by heat. Manufacturers attach VVMs to vials and ampoules of most vaccines. The VVM is printed on the label or cap, or the neck of ampoules of freeze-dried vaccines (Figure 6.7). It looks like a square inside a circle. As the vaccine vial is exposed to more heat, the square becomes darker.
You should only use vaccines where the inner square in the VVM is lighter in colour than the outside circle (Figure 6.8, top row). Vials with VVMs in which the inner square has begun to darken, but is still lighter than the outer circle (Figure 6.8, second row), should be used first, i.e. before vials where the VVM square has not begun to darken. Vials with VVMs in which the inner square matches the colour of the outer square, or in which the inner square is darker than the outer circle, have reached or gone beyond the discard point and should not be used (Figure 6.8, bottom two rows).
VVMs respond to heat — but not to freezing!
VVMs respond to heat — they do not measure exposure to freezing temperatures. A vaccine may have been frozen and have lost its potency, but the VVM cannot tell you this. So even if the VVM indicates that the vaccine has not been exposed to heat, the vaccine may still have been frozen. Therefore, for freeze-sensitive vaccines, it is important to establish that they have not been frozen before using them. Inspect the freeze indicator, as described next.