One method to achieve freeze prevention in vaccine carriers is to condition ice-packs that are normally frozen to temperatures as low as -20°C. Conditioning ice-packs involves partially thawing ice-packs in accordance with the World Health Organization’s (WHO’s) procedure EVM-SOP-E7-04. According to the WHO procedure, an ice pack is considered properly conditioned after it has been left outside the freezer for long enough to stabilize at 0°C.

In a preliminary study, it was found that conditioning time depends heavily on separation distance and percentage of shared border with other ice-packs. There was also evidence that conditioning time depends, though to a lesser degree, on the total size of a group of ice-packs conditioned together (including those not immediately adjacent). The data collected indicated that conditioning times for ice packs frozen at –25°C in 20°C to 23°C ambient temperatures can vary by up to 39 minutes, depending on ice-pack configuration. This finding has significance to operating procedures at cold chain points: if operators do not condition ice-packs using a consistent procedure, conditioning time could vary widely and could potentially risk exposing vaccines to freezing temperatures.