For aid agencies who have made a mission of distributing vaccines, the world has become a tangle of electrical cords connecting refrigerators to the most remote villages around the globe.

They call it the Cold Chain. Without it, vaccines would spoil long before reaching their destination and with them considerable expense, effort -- and lives -- would be lost. But it is difficult to maintain the Cold Chain in countries where the power supply is as bumpy as the roads and anything that can go wrong does.

Through the efforts of many players on several continents the chain has been made stronger and its weak links easier to spot. See a case study showing how the Cold Chain controls the distribution of vaccine.

Ideal temperatures Some vaccines are sensitive to heat, others can be damaged by freezing. Each minute and degree above or below the ideal temperature shortens the vaccine's life. Some guidelines:

Monitoring vaccines It is difficult to tell if a vaccine has been exposed to high temperatures because there is no change in its appearance. Safeguards have been developed to warn health workers of vaccine spoilage.
When vaccines reach health clinics and are stored in refrigerators, they are monitored twice a day and readings are recorded on a chart to ensure a safe temperature is maintained.

Vaccine vial monitors
Every vial is also shipped with a temperature-sensitive label, developed by Seattle-based PATH, that health workers monitor during vaccination sessions.

SAFE If the inner square is lighter than the outer ring and the expiration date is valid, the vaccine is usable.

SPOILED If the inner square matches or is darker than the outer ring, the vaccine must be discarded.

The shake test
DPT, hepatitis B and tetanus toxoid vaccines can all be damaged by freezing. By shaking two vials, side-by-side, one that might have been frozen and one that has never been frozen, health workers can determine if a vaccine has spoiled.