Hi Jesse
As you mentioned you could determine the loss of life of a vaccine from the VVM or its MKT temperature history. When looking at the performance of a refrigerator it is easier to use some method based on the temperature history of the refrigerator rather than looking at the change in dot density of a VVM on a particular vial of vaccine. Based on the refrigerators MKT temperature, the performance of a refrigerator can easily be determined. For example if for a one month period the MKT temperature stayed between 2 Deg C and 8 Deg C the life of the life of vaccines would not be adversly effected. It may be that during this one month period there were daily temperature excursions to 12 Deg C however the MKT temperature could still remain below 8 Deg C and the vaccines would not be adversely effected.
Currently in the field the performance is determined by an alarm system, these alarms have little corrolation to the performance of a refrigerator. In the example given the alarm may have gone off 30 times and yet there were adverse effects to the vaccines. When the day/night tests are done the performance of the refrigerators is not determined by the currently used alarm systems but by the MKT temperature.
A small electronic device could easily be built which would directly read the MKT temperature. The device could also read out the loss of life of vaccines of different sensitivities. This device would be of paticular value when determining the performance of a vaccine carrier. For example trips could be extened if some percent loss of life on the journey was acceptable. If for example a vaccine was to be used no longer than several days after its arrival perhaps a 50% loss of life would be acceptable.
For a particular vaccine carrier data could be presented to show how long a journey could be made for a particular % loss of life. This data could be generated during PQS testing, an appropriate test temperature may be 32 Deg C.
Larry
Sun Frost