6.3.2  Freeze indicators

Freeze indicators are devices used to monitor the exposure of vaccines to freezing. Freeze indicators are packed with batches of freeze-sensitive EPI vaccines (pentavalent, PCV10 and TT), as well as with other freeze-sensitive vaccines such as HepB, which may be used to protect healthcare workers. The most commonly used type of freeze indicator is the freeze-tag (Figure 6.9). This is an irreversible temperature indicator that shows if a product, such as a vaccine, has been exposed to freezing. It consists of an electronic temperature measuring circuit with a liquid crystal display (LCD). A small blinking dot of light in the corner of the display shows that the freeze-tag is functioning correctly.

Figure 6.9  Freeze-tags showing: (a) ‘good status’ display; (b) ‘alarm status’ display. (Photo: WHO)

If the freeze-tag is exposed to a temperature below 0^oC (with a range between +0.3 ^oC and –0.3 ^oC) for more than 60 minutes (with a range of between 57 to 63 minutes), the display will change from the ‘good status’ (Figure 6.9a) to the ‘alarm status’ (Figure 6.9b).

Vaccines that have been exposed to freezing may have been damaged and should be checked by using the shake test, as described below.