Getting a vaccine to the last village

AThe hardest part of a vaccination programme is rarely the vaccine. Developing one is expensive and slow and it is also the part that attracts the money and the attention. Moving a dose from a factory to a child in a village four hours from a paved road is cheap in comparison, unglamorous, and the point at which most programmes actually fail.

BThe difficulty has a name. Most vaccines must be kept between two and eight degrees from the moment they are made until the moment they are used, and the unbroken sequence of refrigerators, cold boxes and vehicles that achieves this is called the cold chain. It is a chain in the strict sense: every link must hold, and a dose that has been too warm for too long is not weakened but worthless, and worse than worthless, because it looks exactly the same as a good one.

CThe last part of the journey is where the chain breaks. The route from a factory to a national store is handled by professionals with refrigerated vehicles and is rarely the problem. The route from a district store to a clinic, and from the clinic to a settlement with no electricity, is covered by a health worker on a motorcycle with an insulated box and some frozen packs, and the limit of that arrangement is a few days in a cool season and considerably less in a hot one.

DFreezing is the failure that surprises people. Attention is concentrated on vaccines getting too warm, but several important vaccines are destroyed by being frozen, and in field surveys a large share of doses are found to have been below zero at some point, usually because they were packed against frozen ice packs that had not been conditioned. The remedy is a simple instruction, which is to leave the packs out until water can be heard moving inside them before loading, and the fact that this remedy is an instruction rather than a device is exactly why it is so often skipped.

EWhat has changed the picture most is not a better refrigerator but a sticker. A vial monitor is a small square on the label containing a material that darkens as it absorbs heat, slowly at low temperatures and quickly at high ones, so the square records cumulative exposure rather than a single reading. When the square becomes darker than the printed reference ring beside it, the dose is discarded. It costs a few cents, requires no power, and transfers the decision from a guess to an observation.

FThe remaining problems are organisational and they are stubborn. A clinic with an unreliable supply learns to hold more stock than it needs, which increases the quantity exposed to any failure. A health worker who has opened a ten-dose vial for one child must either waste nine doses or wait for more children, and the policy that follows from each choice is defensible and unpopular. These are not technical questions, and the programmes that do best are the ones that have decided them explicitly rather than leaving each worker to decide alone.