Vaccines lose potency if they get too warm, and in many cases if they freeze. Keeping them within range from manufacture to administration is a logistics problem solved through monitored handovers.
The chain is a series of handovers
A vaccine passes through manufacturing storage, national and regional stores, district facilities and finally a clinic refrigerator, with a transport leg between each stage.
Each stage is individually refrigerated and reasonably reliable. The vulnerable moments are the transitions, where product sits on a loading dock or in a vehicle waiting to be received.
Losses concentrate in those gaps, which is why cold chain design focuses on handover procedures and receiving discipline rather than on the capacity of the storage units.
Freezing is as damaging as heat
Several vaccine types are irreversibly damaged by freezing, and the damage is not visible on inspection, so an affected vial looks identical to an intact one.
Freezing frequently occurs inside equipment intended to prevent it. Ice packs conditioned improperly, or product placed against a cooling surface, can drop below zero while the compartment average looks correct.
Guidance therefore specifies not only a temperature range but how packs are prepared and where product is positioned, because the average reading conceals the extremes that matter.
Monitoring makes the failure visible
Electronic loggers travel with shipments and record temperature continuously, so a receiving facility can see the full history rather than the reading at the moment of arrival.
Vials also carry indicators that change appearance cumulatively with heat exposure, allowing a health worker at the point of use to judge whether that specific vial remains usable.
The two systems answer different questions: the logger identifies where the chain failed, while the indicator determines whether a particular dose should be administered.
Power and equipment set the practical limit
Refrigeration depends on reliable electricity, and much of the world's clinic network operates with interrupted supply, which is the binding constraint on rural coverage.
Equipment designed to hold temperature through outages, including solar-powered units and passive containers rated for extended periods, addresses this at considerable capital cost.
Maintenance is the recurring weakness. A unit that is installed but unserviced eventually drifts out of range, and without monitoring nobody discovers this until stock has been compromised.
Wastage is measured and planned for
Programmes track wastage explicitly and distinguish between doses discarded because a vial was opened and only partly used, and doses lost to temperature excursions.
The two have different remedies. Open-vial wastage is managed through session planning and vial size, while excursion loss is a cold chain engineering problem.
Supply forecasts include an allowance for both, since ordering exactly the number of doses required would guarantee shortages at the clinics with the weakest infrastructure.