Keeping Vaccines Cold: The Unseen Engineering of Immunisation

APublic accounts of immunisation tend to end at the laboratory door, as though the difficulty of protecting a population lay entirely in designing the molecule. In practice a vaccine spends most of its working life in transit, and almost every hazard it meets on that journey is a matter of temperature. The rule governing the journey is deceptively simple. From the moment a batch leaves the filling line until the instant it is drawn into a syringe, it must be held between two and eight degrees Celsius, and any departure from that band is logged as an excursion. Yet the range is not a biological constant. It was fixed in the 1970s as a conservative compromise, wide enough to be met by the domestic refrigerators then available and narrow enough to protect the least stable products on the market, and it has since been written into national regulations that no manufacturer has much reason to reopen. The equipment assembled to honour it, from refrigerated lorries to insulated carriers and ice packs, is known collectively as the cold chain, and a 2018 audit by the Vantry Institute of Thermal Logistics put its share of routine immunisation costs at rather more than a fifth.

BThe popular image of the threat is a lorry stranded in the heat, but the evidence gathered by Naomi Okereke, an engineer at the same institute, points the other way. Between 2015 and 2019 her team fitted continuous loggers to 1,400 consignments moving through eleven national programmes, and found that 31 per cent had at some point fallen below zero, while only 6 per cent had risen above eight degrees. Freezing is the more serious of the two failures. Heat degrades the proteins that teach the immune system what to look for, but it does so gradually and leaves a visible marker on the vial; a single freezing event can shatter the aluminium salts added to sharpen the response, and the vial afterwards looks exactly as it did before. Okereke maintains that the industry has spent four decades solving the wrong half of the problem. Her figures, however, come only from routes on which ice packs are taken straight from a freezer and placed against the cartons without being allowed to warm first. Where that conditioning step is supervised, she concedes, the sub-zero readings largely disappear.

CA second line of attack questions whether the band needs defending at all. Regulators have begun to grant what is called a controlled temperature chain, a licence under which a named vaccine may leave refrigeration for a single defined period near the point of use. The terms are strict. Six products have so far been cleared to travel for up to four days at ambient temperatures reaching forty degrees, provided that the excursion occurs on the final leg of the journey, is not repeated, and ends with a vial monitor still showing an unexpired square. The licence therefore does not so much relax the rule as move it to one closely defined point. Field trials in 2016 recorded no measurable loss of potency under those terms, and campaigns using the licence reached villages that refrigerated transport had never reached at acceptable cost. Uptake nonetheless remains modest. Okereke attributes the hesitancy of ministries to liability rather than to science, since a health department that authorises an excursion assumes a risk it can neither insure against nor easily explain.

DRahel Bekele, who studies public administration at Duns College, is sceptical that either engineering or licensing addresses the constraint that actually binds. What determines whether a child receives a potent dose, she contends, is a decision taken by a health worker holding a vial of uncertain history, and that decision is shaped less by thermodynamics than by what the clinic's records will afterwards show. In a survey of 620 clinics carried out in 2021 her group found that 47 per cent of vaccinators would administer a vial they suspected had frozen rather than declare a loss, because discarded stock has to be accounted for while a weak dose produces no immediate consequence at all. Bekele therefore argues that devices which merely inform the worker are beside the point, and that only a reporting system removing the penalty for honest disposal will alter behaviour. She concedes two weaknesses. The responses were self-reported, and the study was confined to a single country whose supply arrangements are unusually centralised, so the proportion elsewhere could differ considerably.

ESome of the older bottlenecks have quietly disappeared. Refrigerators driven directly by solar panels, which store cold in ice rather than in batteries and therefore survive a week of cloud, had been installed in 47 countries by 2023, and the number of clinics with no working cold storage has fallen sharply. What has not improved at the same rate is the record of what happened inside those boxes. A monitor placed on the outside of a carton tells an inspector about the carton, not about the vial in the middle of it, and the method used to identify a frozen vial depends on comparing it with a sample frozen on purpose, which few clinics keep. The disagreement between Okereke and Bekele is therefore less about mechanism than about sequence. Okereke insists that the physical failure must be removed before any reporting can be trusted, while Bekele holds that a system nobody reports honestly will conceal whatever failures remain, however rare they become. Both accept that the cold chain has turned into a problem of information as much as one of refrigeration.