Keeping vaccines in range, from depot to last mile

SDD units store cold energy directly — typically in an ice-lined or water-pack jacket — rather than electrical energy in a battery. In field use the battery is the part that fails first: lead-acid and even lithium banks degrade on a fixed timeline, and replacement supply chains rarely reach the facilities that need them. WHO PQS prequalifies SDD designs for exactly this reason. Battery-buffered units still have a role where deep holdover or ancillary loads are required, but they carry a maintenance liability the procurement officer must budget for from day one.

In many monitored chains, yes. Heat exposure is intuitive and well-watched; freezing is silent and frequently caused by the cold-chain equipment itself — ice packs frozen solid and placed against vials, or refrigerators set too low. Freeze-sensitive vaccines including DTP, tetanus toxoid, hepatitis B, Hib, and IPV can lose potency irreversibly after a single freezing event, with no visible change to the vial. Published reviews have found freeze exposure across the great majority of full distribution chains studied. This is why we treat freeze protection as a named criterion rather than a footnote.

PQS prequalification means the equipment has met a defined Performance, Quality and Safety standard under controlled testing — holdover and cold-life times, freeze prevention, energy performance, and so on. It is a strong floor and the right starting filter. It does not, however, capture serviceability in a specific country, spare-part availability, or how the unit behaves after three years of dust and brownouts. Our reviews start from the PQS list and add the field-durability judgement the standard cannot.

Monitoring fails on the human layer, not the hardware. The durable pattern pairs an electronic 30-day logger that alarms locally with a simple single-use freeze indicator that needs no download, plus a fixed, written read-and-record routine tied to a role rather than a named individual. Loggers that require a laptop, a cable, and trained software use are the ones found unread. Our campaign-monitoring coverage treats the SOP and the alarm-response drill as equipment in their own right.

They should, and most failures at the intersection come from treating them separately. Several point-of-care diagnostic assays — particularly molecular CD4 and viral-load cartridges — carry their own refrigeration limits, adding a second cold chain on top of the vaccine one and competing for the same finite passive-cool-box capacity. A logistician who budgets cold space for vaccines alone can find a diagnostics consumable silently voided by a heat excursion no one was watching. Plan the two together, and give the diagnostics consumables their own dedicated logger rather than borrowing a reading from the vaccine fridge. Our field-diagnostics coverage treats this as the first question to settle before committing to a platform.

No, and this is the trade-off field teams flag most. A theoretically superior unit with no in-country parts pipeline becomes the inferior choice the moment a gasket, controller, or panel fails and the replacement is months away. The decisive variables in a low-resource setting are serviceability and the spare-part supply chain, not the headline spec. A modestly specced, PQS-prequalified unit that a trained local technician can repair within days will, in practice, hold the chain better than a premium unit waiting on a component that never ships. Match the spec to the climate and catchment, then weight serviceability heavily.