Taking stock

摘要

KEEP a tomato cool in a refrigerator and it will stay fresh far longer than it would at room temperature. Accidentally freeze it, though, and you will reduce it to a disgusting mush. A similar problem plagues the storage of vaccines. About six in ten of those procured by unicef, the un's children's fund, must be stored at a temperature between 2°C and 8°C. Generally, the focus of efforts to do this is on the top end of the range, with the establishment of "cold chains", the links of which are refrigerators on the journey from factory to clinic, to stop vaccines overheating. Less effort is put into making sure a vaccine never gets too cold. But a vial of vaccine that has been accidentally frozen, and then thawed, may lose its potency as surely as one that has been warmed up.