After reading Umit's book my interest in what I call the "buffered Cold Chain" BCC, was renewed.In his book I found the history and the description of the obstacles which were overcome very interesting.
By BCC I mean a cold chain which will maintain temperatures between 20 degC and 25 degC . These higher temperatures could be maintained with simple low cost systems. The life of a vaccine will be shortened at these higher temperatures however in many instances the reduced life will be adequate. The time to end point at these temperatures can be determined form the the graphs on pg 183 of Umit's book. At 20 degC the life of a VVM 2 vaccine will be 22 days and the life of a VVM 7 will be considerably longer 89 daysand the life of the less sensitive vaccines will be even longer.
An interesting point is that a 5 degC change in temperature will change the life of all
vaccines by a factor of aproximately two, this can be seen form the graphs on pg 183 of Umit's book. For example if the storage temperature of a VVM 14 vaccine increases from 15 degC to 20 degC it's life will decrease from about 400 days to 200 days, for a VVM2 vaccine if the storage temperature increases from 20 degC to 25 degC the life of the vaccine will decrease from about 20 days to 10 days.
Storage temperatures in the 20 degC to 25 degC are easily attained by thermal electric cooling.These solid state devices are reliable, low in cost and light weight, unfortunately at lower temperatures
they are very inefficient. However in the 20 degC to 25 degC temperature range they can provide reliable refrigeration.
A thermal electric cooled refrigerator can be purchased for less than $100 and with slight modifications the cooler could be connected directly to a solar module.The installed system cost of SDD thermal electric cooler would be about one fifth the cost of a standard compression driven SDD system. The thermal storage required to maintain adequate temperatures at night will be considerably less than what is required in standard SDD refrigerators because the storage temperature will be close to the night time temperature.
Thermal storage requirements could be further reduced by incorporating "Time Temperature Integration"in the monitoring device. On pg 150 of Umit's book he points out how MKT temperatures can fulfill this function. The MKT temperature is a weighted average which counts higher temperatures more than lower temperatures.Details of how MKT temperature is used to determine the impact of a temperature excursion are detailed in the Science section of Umit's book. If for example at night the temperature climbs to 28 degC,and during the day the cooling system reduces the temperature to 12 degC the MKT temperature may be about 22 degC; with this temperature excursion the life of the vaccine willbe the same as a vaccine which is constantly kept at 22 degC. A small electronic device could be built which would display the MKT temperature and also the loss of life of the stored vaccines. The loss of life could be displayed for vaccines of different sensitivities. the results displayed would be consistent the change of life indicated by VVM's.
The Buffered Cold Chain is another tool to expand the cold chain in a low cost manner.
If there is further interest in this concept, please contact me.
Larry Schlussler Phd