In May 2006, an earthquake near Yogyakarta, Indonesia, knocked out electricity to health clinics for several days. The refrigerators holding vaccine vials went warm, and before 1996 a vaccine like that was too often thrown out, or, worse, sometimes used anyway, since nothing on the vial itself could tell a health worker whether a given dose had taken too much heat. By 2006 a small sticker on each vial could. Health workers checked it, confirmed that most of the vaccines were undamaged despite the heat, and saved roughly 50,000 doses that would otherwise have been thrown away.
The World Health Organization founded the Expanded Programme on Immunization in 1974 to get vaccines to children worldwide, and ran it for more than two decades with a hole at the center of it. From 1974 to January 1996, WHO and UNICEF later wrote, there were no means for the health worker to know whether a given vial had been exposed to more heat than the vaccine inside it could survive. Agencies compensated by overbuilding the cold chain itself, with costly refrigeration and strict handling rules, and still had no way to check the vial actually in a worker's hand.
John Lloyd, an architect turned health-program designer, spent the mid-1970s learning why the cold chain kept failing. Working with Ghana's new immunization program, he found that refrigerators and picnic coolers built for European kitchens could not survive 40°C heat, long dirt-road supply routes, and spare parts that were hard to find once something broke. By the late 1970s, Lloyd and other WHO consultants turned to a different fix, working with the nonprofit PATH and, from the early 1980s, the Temptime Corporation, to build a label that reacted to heat the way the vaccine beneath it did. From January 1996, oral polio vaccine supplied through UNICEF carried the monitors as standard.
Nobody took the sticker's word for it. Between 1981 and 1992, prototype monitors were field-tested in nineteen countries, interviewing more than 170 health workers and running 89,700 labels through 1,432 health centres. In 1997, a scientist at Britain's National Institute for Biological Standards and Control independently tested whether the color change actually tracked the vaccine's own potency; in 1999, a British consumer-testing laboratory separately tested the monitors' performance against WHO's own specifications and confirmed they met them. Field studies in eight countries, measuring what the sticker did for real campaigns, let health workers carry polio vaccine beyond the reach of a working refrigerator; those results went to the Technical Network for Logistics in Health's 1998 meeting, which recommended putting monitors on every vaccine, not polio's alone.
A dose of vaccine ruined by heat looks exactly like one that works: nothing about it warns the person giving the shot. For twenty-one years, nothing on an EPI vial told the health worker holding it which kind it was, and the programme spent its way around that gap with costly refrigeration equipment and strict handling rules. The sticker did not make a vaccine sturdier, and it still cannot tell a freeze-damaged dose from a good one. What it made visible was narrower and just as useful: whether this dose had already taken more heat than it could survive, something a worker could now see on the vial instead of assuming.