The effect of cool water pack preparation on vaccine vial temperatures in refrigerators

Abstract

Cool water packs are a useful alternative to ice packs for preventing unintentional freezing of vaccines during outreach in some situations. Current guidelines recommend the use of a separate refrigerator for cooling water packs from ambient temperatures to prevent possible heat degradation of adjacent vaccine vials. To investigate whether this additional equipment is necessary, we measured the temperatures that vaccine vials were exposed to when warm water packs were placed next to vials in a refrigerator. We then calculated the effect of repeated vial exposure to those temperatures on vaccine vial monitor status to estimate the impact to the vaccine. Vials were tested in a variety of configurations, varying the number and locations of vials and water packs in the refrigerator. The calculated average percentage life lost during a month of repeated warming ranged from 20.0% to 30.3% for a category 2 (least stable) vaccine vial monitor and from 3.8% to 6.0% for a category 7 (moderate stability) vaccine vial monitor, compared to 17.0% for category 2 vaccine vial monitors and 3.1% for category 7 vaccine vial monitors at a constant 5 °C. The number of vials, number of water packs, and locations of each impacted vial warming and therefore percentage life lost, but the vaccine vial monitor category had a higher impact on the average percentage life lost than any of the other parameters. The results suggest that damage to vaccines from repeated warming over the course of a month is not certain and that cooling water packs in a refrigerator where vaccines are being stored may be a useful practice if safe procedures are established.

Keywords: Cold chain; Cool water pack; Freeze-sensitive vaccine; Supply chain; Vaccine; Vaccine vial monitor.

Fig. 1

Front and top views of each test setup.

Fig. 1

Front and top views of each test setup.

Fig. 2

Process for calculating loss of vaccine vial monitor (VVM) life. Slopes of reaction rates at different temperatures (a), calculated from the lifetime-temperature points established by the World Health Organization, are plotted to calculate the Arrhenius curve for that VVM (b). Values shown here are for VVM2 at the center of the 90% tolerance range for reaching VVM endpoint. The calculated Arrhenius equation is applied to a warming curve from a tested vial (c) to create the instantaneous life-lost curve (d). The instantaneous values can be summed to find the cumulative life lost over 24 h (d).

Fig. 3

Loss of VVM life over 30 days using maximum observed vial warming (Setup 8).

Fig. 4

Heat map of vaccine vial monitor category 2 (VVM2) life lost in percentage over 30 days for three different test setups. Each setup contained 10 vials on a shelf and 8 water packs. Increasing the space between vials increased the range of VVM life lost. Vials closer to the water packs and farther from the walls lost more life. ¹ The thermocouple in this location was discovered not submerged in water inside the vial; therefore, these data have been omitted.