The term "cold chain" describes the techniques and procedures used to ensure that heat-sensitive biological products such as vaccines, sera, and antibiotics do not deteriorate in transit from the place where they are produced to the patients and others who ultimately receive them as part of a preventive or therapeutic regimen. Some important varieties of vaccine and sera are inactivated even at room temperature in temperate zones, and in hot tropical regions they are not only inactivated, but some may even become toxic. Maintenance of an intact cold chain is particularly necessary in mass vaccination programs in tropical regions, where diseases such as measles and poliomyelitis can do much harm to vulnerable infants and children. Heat-sensitive vaccines and other biological products must be refrigerated immediately after they are produced; and they must stay refrigerated while in transit to distribution centers and to peripheral clinics where the target populations are located. During vaccination sessions the stored vaccines must be kept at a suitable low temperature at all times, and only enough for use on people actually awaiting vaccination should be removed from refrigeration at one time. Vaccines should be distributed to small peripheral clinics in insulated polystyrene boxes packed in dry ice. When vaccine failure is detected, either by serological studies of vaccinated populations or by the occurrence of cases in a supposedly vaccinated population, the cause is most likely to be a breakdown in the cold chain; and this most often happens at the periphery of the chain.
John M. Last