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Thermal Analysis of Refrigeration 

Systems Used for Vaccine Storage

Michal Chojnacky

Wyatt Miller 

Dean Ripple 

Gregory Strouse

NISTIR 7656
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NISTIR 7656

Thermal Analysis of Refrigeration 

Systems Used for Vaccine Storage

Michal Chojnacky 

Wyatt Miller 

Dean Ripple 

Gregory Strouse

CSTL 

November 2009

U.S. Department of Commerce 

Gary Locke, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Deputy Director 
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ABSTRACT 

Each year, billions of dollars of vaccines are stored in refrigerators at the facilities of a variety of 

medical providers. Many vaccines must be maintained in the range 2 °C to 8 °C to retain product 

potency. We have tested the performance of two types of household refrigerators to determine if 

these refrigerators are suitable to this task, and to identify proper storage and temperature 

monitoring methods. Nineteen calibrated Type T thermocouples, distributed through the 

refrigerator interior, served as reference thermometers. Attachment of thermocouples directly to 

vaccine vials gave accurate measurements of the vaccine temperature, which often differed from 

the air or interior wall temperatures during door openings or defrost cycles. A household, fullsize freezerless refrigerator [capacity = 0.473 m3

 (16.7 cu. ft.)] proved fully adequate at 

maintaining vial temperatures within the desired 2 °C to 8 °C range, independent of how the 

refrigerator was loaded. Tests of intermittent and continuous door opening and of simulated 

power outages demonstrated the value of adding water bottles to the door as a thermal ballast. 

The performance of compact, dormitory-style refrigerators suffered from drift of the refrigerator 

set point, sensitivity to load density, and high temperature non-uniformity. These problems 

make the dormitory-style refrigerator [capacity = 0.077 m3

 (2.72 cu. ft.)] unsuitable for vaccine 

storage. We tested four electronic data loggers as a means of continuously logging refrigerator 

temperatures. Properly located, data loggers accurately monitored vial temperatures for 

extended periods. 

1. INTRODUCTION 

Each year, millions of dollars worth of vaccines are lost in the United States due to improper 

storage conditions. In order to maintain potency, many common vaccines must be kept 

refrigerated between 2 °C to 8 °C. When vaccines are exposed to temperatures above or below 

this range, their potency is reduced until they eventually become completely ineffective. 

A number of factors influence whether vaccines stored in a refrigerator remain within the 

prescribed temperature range. Vaccine storage trays or boxes can obstruct refrigerator air flow, 

creating areas pockets of colder or warmer air. Vaccines stored in certain locations, such as near 

the refrigerator cooling unit, may be kept significantly colder than if placed in the main body of 

the refrigerator. Some factors may vary drastically between different refrigerator types (e.g. 

dorm style, freezerless, dual zone or pharmaceutical grade). Parameters such as temperature 

control stability, air circulation patterns, defrost cycles, and long-term drift of the temperature set 

point can play a major role in determining whether a refrigerator maintains suitable vaccine 

storage conditions. Because of this, simply setting a refrigerator to a temperature between 2 °C 

to 8 °C may not actually result in stored vaccines being kept within that temperature range. 

Accurate monitoring of vaccine temperatures requires attention to the type of monitoring system 

used and its installation in the refrigerator. Different types of temperature monitoring systems 

can influence both the accuracy of the temperature measurement and the ease of use. Depending 

on the placement of a temperature monitoring device, as well as the time or frequency that its 

readings are recorded, the resulting temperature readings may or may not correspond to actual 

vaccine temperatures. 
 Page 4 of 41 

In this study, we explored a number of these factors in an attempt to determine which have the 

greatest impact on vaccine temperature. From this, we can make experimentally verifiable 

recommendations regarding the best and worst vaccine storage configurations and temperature 

monitoring setups. We used an array of calibrated thermocouples, mounted in multiple 

locations, as reference thermometers. An automated data-acquisition system measured the 

thermocouple temperatures continuously, for a wide variety of operating conditions. Attaching 

thermocouples directly to vaccine vials gave temperature measurements highly indicative of the 

actual vaccine temperatures. 

We report measurements for two refrigerator styles: a) compact, “dormitory-style” refrigerator 

[capacity = 0.473 m3

 (16.7 cu. ft.)], and b) a household refrigerator [capacity = 0.077 m3 (2.72 

cu. ft.)] with no freezer unit. Our tests demonstrate that the dormitory-style refrigerator cannot 

reliably maintain vaccine temperatures in the range 2 °C to 8 °C. Weaknesses of the dormitorystyle refrigerators include: poor long-term stability; sensitivity to loading conditions; and large 

temperature non-uniformities. The freezerless household refrigerator, on the other hand, can 

easily maintain vaccines at appropriate temperatures with only a few simple precautions. 

A similar study exploring the suitability and performance of pharmaceutical grade refrigerators 

for vaccine storage was conducted by the Test Research Group for the Australian General 

Practice Network [1]. For further information on vaccine storage practices, including official 

storage guidelines from the Centers for Disease Control, consult references 2 - 5. 

2. EXPERIMENTAL METHOD 

Prior to this study, members of the NIST Thermometry Group visited seven Vaccines for 

Children (VFC) clinics in order to observe their vaccine storage practices. In our experimental 

trials, we attempted to replicate the range of vaccine storage conditions witnessed at VFC clinics. 

This range included variations in loading density. For example, during influenza season, a large 

influx of patients could leave a clinic with dwindling vaccine supplies. At this time, their storage 

refrigerator would probably contain a low packing density. Other times, such as immediately 

following a large vaccine shipment, the same storage refrigerator might be filled to maximum 

storage capacity with boxes of vaccines. This would be considered a high density load. By 

testing a variety of low, medium, and high packing densities, we were able to quantify how this 

type of normal inventory fluctuation affects refrigerator performance. 

We also experimented with different packaging styles. Many of the clinics surveyed used plastic 

trays or bins to organize and store their vaccine inventory. We also observed vaccines stored 

inside cardboard shipping boxes, as well as in their original light cardboard packaging. The 

Centers for Disease Control and Prevention (CDC) guidelines advise storing vaccines in plastic 

trays rather than uncontained on shelves or inside shipping boxes. However, during very busy 

times, clinic workers might temporarily store vaccines in their shipping containers until they can 

be organized into appropriate bins. As a result, we investigated whether these different 

packaging methods would affect the refrigerator’s ability to maintain the proper vaccine storage 
 Page 5 of 41 

temperatures. In this study, we performed trials using three different packing styles: 1) vaccines 

stored exclusively in plastic trays; 2) vaccines stored inside various boxes, such as cardboard 

shipping boxes or their original thin cardboard packaging; and 3) vaccines stored in a mixture of 

plastic trays and cardboard boxes. 

In our survey of VFC clinics, we noted multiple locations storing water bottles in refrigerator 

doors or in the main space of the refrigerator, a practice recommended by the Centers for Disease 

Control. This is because the additional thermal mass of the water bottles should act as a 

temperature ballast, lessening the impact of potential temperature fluctuation sources. To test the 

effectiveness of this practice, we performed some trials with water bottles filling the refrigerator 

door, and some trials without. The typical volume of water bottles used was equal to 3% to 5% 

of the total refrigerator capacity. 

We performed a cycle of measurements on two different refrigerators (a compact, dormitory 

style and a full-size, single-door freezerless style) using permutations of the variables described 

above. Prior to the fist cycle of measurements, the refrigerators were adjusted to give a storage 

temperature of 4 °C. A complete listing of the different trials and order in which they were 

performed is contained in the measurement matrix (Table 1) shown below. After completing the 

trials with varying load densities, packing styles, and with/without water bottles, we performed 

some additional trials designed to simulate some of the different conditions that a VFC clinic 

might experience. 

During the first cycle of trials, the refrigerator door was kept closed. However, in every day use, 

it is not possible to keep a refrigerator door closed at all times. The door must be opened briefly 

in order to add and remove vaccines for use. To mimic the opening and closing of the 

refrigerator door that occurs with normal use, we performed trials in which the door was opened 

at 15 minute intervals for 15 seconds at a time, over the course of at least 2 h. We carried out 

these measurement using at least two different packing schemes (one with water bottles and one 

without) for each refrigerator. We also recorded data with the refrigerator door left ajar for 

approximately 1 h, in order to observe what occurs if a clinic employee forgets to completely 

close the door. 

Another non-idealized but common experience at VFC clinics is fluctuation in the storage room 

temperature. Poor HVAC control, inclement or seasonal weather changes, and planned or 

unplanned outages can all cause unexpected room temperature fluctuations. We wished to 

determine whether these types of changes affect a refrigerator’s ability to maintain the required 

2 °C to 8 °C temperature range. To do this, we recorded refrigerator temperature data while 

increasing the ambient room temperature by several degrees. As before, these trials were 

repeated for each refrigerator using at least two different packing schemes. 

In the final set of trials, we observed what occurs to refrigerated vaccines stores in the event of a 

power outage. In these trials, the refrigerator was set up to run normally. After starting the 

measurement collection system, the refrigerator was unplugged and data collection continued 

overnight. 
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Table 1. Matrix of measurements performed for each refrigerator. 

Trial Water Bottles Measurement Parameters

Low Medium High Trays Boxes Mixed

1 x x normal

2 x x normal

3 x x x normal

4 x x x normal

5 x x x normal

6 x x x normal

7 x x x normal

8 x x x normal

9 x x x normal

10 x x x increase room temp

11 x x x periodic door opening

12 x x x power off

13 x x normal

14 x x increase room temp

15 x x periodic door opening

16 x x power off

17 x x normal

18 x x periodic door opening

Load Density Packing Style

Note: During the dorm-style refrigerator tests, we moved 4 thermocouples from other locations to vials 

starting from 5th trial (in blue). 

To determine the temperature distribution throughout the entire refrigerator space, we arranged 

19 Teflon-insulated, Type T thermocouples in various regions in the refrigerator body. The 

thermocouples were attached to refrigerator walls or storage trays, hung in air, mounted inside of 

glycol-filled bottles, or most importantly, taped to actual vaccine vials. The most important 

feature of a refrigerator used for vaccine storage is its ability to maintain a vial temperature 

between 2 °C to 8 °C. Therefore, monitoring actual vial temperature in addition to other 

locations inside the refrigerator enabled us to effectively evaluate each refrigerator’s 

performance. The installation pattern also allowed us to determine how well thermometers 

placed in different areas match the actual vaccine temperatures. This helped us produce useful 

guidelines for the best placement of refrigerator temperature monitoring devices. 

The thermocouple reference junctions were maintained at 23 °C ±0.002 °C in an auxiliary stirred 

water bath. Prior to use, we calibrated each thermocouple at the melting point of ice (0 °C) by 

placing the thermocouple measuring junction in a Dewar flask containing crushed ice and 

distilled water. The flask itself rested inside the refrigerator under study. An automated, 6 1⁄2 

digit voltmeter monitored the voltage produced by each thermocouple. 

In addition to the 19 thermocouples used to monitor temperature, we tested several different 

electronic temperature loggers throughout the course of this study. At present, VFC clinic 

workers are required to check a thermometer placed inside the refrigerator at certain intervals 

each day. Barring any inaccuracies inherent to the device or poor placement of the thermometer, 

this type of monitoring is still likely to be insufficient. Refrigerator temperature is not constant 
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over time, so a simple thermometer check once or twice a day will not give an accurate 

indication of whether the vaccines have remained within the designated temperature range. 

Additionally, inexpensive electronic data loggers (e.g. self-contained temperature recording 

devices) may provide a useful alternative giving 24-hour-a-day monitoring of refrigerator 

temperature. To investigate the utility of data loggers, we placed three different data loggers 

inside the refrigerator throughout the study in order to compare their performance to that of the 

thermocouples. The placement of the thermocouples and data loggers was kept consistent for the 

duration of trials in each refrigerator. The thermometer installation patterns for each refrigerator 

are shown in Figs. 1 and 2 below. 

Table 2 summarizes the equipment used for this study. Table 3 contains the uncertainties 

corresponding to the measurement devices and systems used. 

Fig. 1. Thermocouple and data logger installation pattern—dorm style refrigerator. 
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Fig. 2. Thermocouple and data logger installation pattern—freezerless refrigerator. 

Table 2. Equipment used for the present study. 

Equipment name: Specifications: 

Dormitory-style refrigerator  Compact refrigerator (120 V) 

 Height: 0.665 m [26.1875 in.] 

 Width: 0.479 m [18.875 in.] 

 Depth: 0.486 m [19.125 in.] 

 Capacity: 0.077 m3 [2.7 cu. ft.] 

 No forced convection 

Freezerless refrigerator  Automatic defrost single-door refrigerator (120 V) 

 Height: 1.65 m [65.125 in.] 

 Width: 0.813 m [32 in.] 

 Depth: 0.676 m [26.625 in.] 

 Capacity: 0.473 m3 [16.7 cu. ft.] 

 Forced convection of cooling air 

Plastic trays  Stacking plastic storage trays/ bins 

19 thermocouples  Type T (copper-constantan) 
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 0.5 mm diameter, 1.83 m length 

 Teflon insulation 

 Calibrated at 0 °C 

Data logger A  Rugged high-temperature logger 

 Metal housing with integral 2.6 cm rigid probe 

 Temperature range: –40 °C to +150 °C 

 Temperature resolution: 0.05 °C 

 Calibrated accuracy: ± 0.5 °C (from manufacturer) 

 Memory: 32,767 readings 

 Battery life: 1 year 

 Body dimensions: 12.2 cm x 2.6 cm dia. 

 Probe dimensions: 2.6 cm x 0.5 cm dia. 

Data logger B  Plastic housing with integral LCD display 

 Temperature range: –35 °C to 80 °C 

 Temperature resolution: 0.5 °C 

 Accuracy: ± 0.5 °C (manufacturer rating) 

 Memory: 16,328 readings 

 Battery life: 1 year 

 Nominal body dimensions: 12.6 cm x 2.4 cm max dia. 

Data logger C  Plastic-housing temperature and humidity Logger 

 Temperature range: –35 °C to 80 °C 

 Temperature resolution: 0.5 °C 

 Accuracy: ± 0.3 °C (manufacturer rating) 

 Memory: 16,328 readings 

 Battery life: 1 year 

 Nominal body dimensions: 10.2 cm x 2.3 cm max dia. 

Data logger D  Data logger with flexible temperature probes 

 Temperature sensors in readout unit and in a flexible 

probes 

 Temperature range (readout): –40 °C to 85 °C 

 Temperature range (flexible probe): –15 °C to 105 °C 

 Temperature resolution: 0.1 °C 

 Accuracy: ± 0.5 °C (manufacturer rating) 

 Memory: 2000 readings per channel 

 Battery life: 2 years 

 Nominal body dimensions: 7.6 cm x 7.5 cm x 2.3 cm 

Thermocouple measurement 

system 

 6.5 digit multimeter with scanner 
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Table 3. Measurement Device Uncertainties 

Device name: U(k=2), °C 

Thermocouple measurement system 0.12 

Data logger A 0.58 

Data logger B 1.41 

Data logger C 0.67 

Data logger D 0.59 

In general, each trial was started during the afternoon, and measurements recorded overnight and 

into the next morning. On average, we recorded 15 to 19 h of data for each trial, although some 

trials were continued over the course of several days. The only exception to this was the door 

opening trials, which only lasted 3 to 4 h. The thermocouple measurement system records 

readings once every 10 seconds. The individual data loggers can be set to various reading rates. 

In general, the loggers were set to record once every minute. However, during the door opening 

trials, we increased the reading rate to once every 10 to 42 seconds (depending on the 

capabilities of the logger). If we planned to record data for a very long time, such as over the 

course of a long weekend, the loggers were set to record data every 2 min. This was done to 

avoid running out of memory partway through the test. 

3. INTERPRETATION OF RESULTS 

3.1 Freezerless Refrigerator 

3.1.1. Effect of loading density 

In trials performed using this refrigerator model, vaccine storage density did not appear to have a 

major impact on the refrigerator’s capability to maintain the desired temperature range. The 

photographs below (Fig. 3) show the various levels of packing density that were tested. 

 

 Low Density Pack Medium Density Pack High Density Pack 
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Fig. 3. Typical packing patterns. 

Throughout the course of the study, average thermometer temperatures remained within 

approximately a 2 °C range regardless of whether the refrigerator load was low, medium, or high 

density. All of the thermometers maintained temperatures within the prescribed 2 °C to 8 °C 

range, with the exception of one thermometer (15) attached to the back of a storage tray (and 

thus not a good indicator of vial temperature), which read an average of 1.8 °C during one trial. 
 Page 12 of 41 

0

1

2

3

4

5

6

7

8

empty

lowtrays

medtrays I

medtrays II

medtrays bottles

medboxes bottles

highboxes bottles

lowmixed bottles

medmixed bottles

highmixed bottles

highmixed bottles opendoor

medmixed

medmixed opendoor

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top) Fig. 4. Average temperature of freezerless refrigerator for all sensors, as a function of loading 

pattern. Labels on the abscissa correspond to entries in the measurement matrix of Table 1. 

 

Figure 4 shows the average reading for each thermometer across all of the closed-door 

freezerless refrigerator trials, as well as the periodic door-opening trials. It is readily apparent 

that the main body of the refrigerator as well the vaccine vials themselves stayed within the 

prescribed temperature range, regardless of packing density variations. Considering a 

refrigerator used for everyday vaccine storage, the arrival of a large vaccine shipment or the 

onset of influenza season is likely to result in a sudden increase or decrease in the amount of 

material stored in the refrigerator. Because these types of load density variations do not greatly 

affect overall temperature stability, the freezerless refrigerator model appears to be suitable for 

normal vaccine storage. 

3.1.2. Effect of packing style 

As with the loading density variation, changing the vaccine packing style did not have a major 

impact on the overall refrigerator and vial temperature stability, whether plastic trays, cardboard 

boxes, or a combination of both were used to store the vaccines. (See Fig. 5.) While official 

guidelines for vaccine storage mandate that vials be stored in permanent trays or containers and 

organized by type, we observed other storage conditions during our visits to the VFC clinics. 

For example, a busy clinic with a sudden influx of vaccines might temporarily store vaccines 

inside their shipping boxes. The measurement results indicate that the freezerless refrigerator 
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model would be able to withstand this type of practice, continuing to preserve the prescribed 

vaccine storage temperature range regardless of packaging style. 

Fig. 5. Various vaccine storage methods. 

3.1.3. Effect of door opening and closing—normal use simulation 

The results of the periodic door-opening trials also support the efficacy of the freezerless 

refrigerator model. Although thermometers attached to refrigerator walls and hanging in the air 

recorded temperatures above the allowed maximum temperature of 8 °C, all thermometers 

attached to vials remained within the desired range in spite of the repeated exposure to room 

temperature air. 
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0

2

4

6

8

10

12

14

16

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

Fig. 6. Door opening trial for the freezerless refrigerator using a medium density mixed load with 

no water bottles. 

As expected, the results shown in Fig. 6 above show a spike in temperature occurring each time 

the refrigerator door was opened. The majority of the thermometers recorded an overall higher 

average temperature during this trial than during the closed door test using the same packing 

scheme. Three thermometers, all of which were placed close to the refrigerator cooling unit, 

recorded a slightly lower temperature during the door opening trial. This is most likely due to 

the refrigerator pumping out additional cold air to compensate for the warm air introduced during 

door opening. 

It is important to note that the thermometers attached to the refrigerator walls and hanging in the 

air were the only ones to show dramatic spikes exceeding the 8 °C limit. In contrast, the 

thermometers attached to vials, which recorded smaller temperature increases in response to the 

door opening, remained within the prescribed temperature range of 2 °C to 8 °C for the duration 

of the test, as shown in Fig. 7 below. 
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Medium Density Mixed Load Without Bottles

1

2

3

4

5

6

7

8

9

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45

Duration of measurement, h:min

Temperature, °C

6 (vial - floor)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

High Density Mixed Load With Bottles

1

2

3

4

5

6

7

8

9

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45

Duration of measurement, h:min

Temperature, °C

6 (vial - floor)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

Fig. 7. Door-opening trials for the freezerless refrigerator using two different loading 

configurations, with and without water bottles. Only thermocouples monitoring vial 

temperatures are shown. 

Fig. 7 compares the data recorded during the medium density, mixed loading scheme without 

water bottles trial to that of the high density mixed scheme with water bottles trial. The data 

from the second trial (with bottles) shows similar, but slightly smaller temperature fluctuations. 

While it is difficult to determine whether the water bottles made a difference during the closed 

door tests, the data from the two door-opening trials—with and without water bottles—supports 

the assertion that bottles stored in a refrigerator door help to reduce temperature fluctuations. 

To further evaluate the water bottles’ effectiveness as a temperature stabilizer, we compared the 

temperature increase of the thermometers across the two separate trials. In general, the average 

temperature recorded during the door opening test was higher, except in the case of a few 

thermometers near the cooling unit, as discussed previously. As always, the most important 

features to examine are the temperature increases of the actual vials. Table 4 below shows the 

different door opening temperature changes for the two trials, with and without water bottles. In 

this case, the temperature change is defined as the difference between the average temperature 

recorded during the closed door trial and the average temperature from the door-opening trial 

with the same loading configuration. 
 Page 16 of 41 

Table 4. Temperature changes during door-opening tests (all values in degrees Celsius). 

Sensor High Density Mixed Medium Density Mixed

Name/Location With Bottles No Bottles

1 (top wall) 0.00 0.57

2 (mid wall) 0.37 1.12

3 (bottom wall) 0.91 1.32

4 (top back wall) -0.20 0.50

5 (air) 0.42 0.95

6 (vial - floor) 0.48 0.83

7 (air) 0.34 1.52

8 (air) 0.49 1.51

9 (air - top) -0.13 0.69

10 (in box) 0.21 1.16

11 (in box) 0.16 1.11

12 (vial - mid) 0.07 0.62

13 (vial - mid) -0.06 1.11

14 (inside tray) 0.53 1.07

15 (back of tray) 0.41 1.03

17 (glycol - floor) 0.65 0.88

18 (glycol - mid) 0.14 0.97

19 (glycol - top) -0.17 0.79

20 (vial - low) 0.30 1.34

MT (floor) 1.05 1.13

SU (mid) 0.23 1.28

TD (glycol - top) -0.04 0.43

The vial data is highlighted in boldface colored font. All of the thermometers recorded a smaller 

temperature increase when water bottles were used in the door. Without water bottles, the 

periodic door opening resulted in an increase of 0.2 °C to 1.2 °C higher than that with water 

bottles. While this is not a huge effect, it is enough to recommend widespread use of water 

bottles as a ballast in vaccine storage refrigerator doors. In particular, if a refrigerator’s average 

temperature happened to be operating on the higher side of the prescribed range, an extra degree 

of protection against temperature fluctuation due to normal door opening could be the difference 

between a viable vaccine and a ruined vaccine. 

The effect of the water bottles was even more dramatic during the test in which the refrigerator 

door was left cracked open for 1 h, to simulate an employee forgetting to completely close the 

door. Table 5 below shows the amount of time it took once the door was opened for each of the 

vials to exceed the allowed maximum temperature of 8 °C. 
 Page 17 of 41 

Table 5. Time after opening door until vial temperature exceeded 8 °C. 

High Density Mixed Medium Density Mixed

Name and Location With Bottles No Bottles

(minutes) (minutes)

vial 20 (on top of box) 3 1

vial 13 (in tray) 12 2

vial 10 (inside original packaging) 20 9

vial 11 (inside original packaging) 25 9

vial 6 (inside cardboard box) 32 17

vial 12 (inside cardboard box) 46 49

The window of time before the vaccines exceeded the allowed temperature was significantly 

longer when the water bottles were in place. Depending on how the vaccines are stored, water 

bottle ballast can provide an extra 2 min to 15 min of leeway before vaccines are subjected to 

out-of-range temperatures. Storing the vaccines inside their original packaging or in a cardboard 

box provides a further layer of protection from the warm outside air. 

3.1.4. Effect of Increasing Room Temperature 

Although vaccine storage refrigerators should be kept in a fairly well temperature controlled 

room, considerable ambient temperature fluctuations may result from planned HVAC outages at 

night or weekends, or from unplanned circumstances such as an AC outage or poor thermostat 

control. To determine whether these fluctuations impact the freezerless refrigerator’s ability to 

maintain its set point temperature, we performed three trials in which the room temperature was 

increased 3 °C to 5 °C above the ambient operating temperature of 21 °C to 22 °C. 

Data logger D has a thermometer included in the main readout unit as well as an additional probe 

attached via cable. This probe was inserted into a glycol-filled bottle kept inside the refrigerator 

throughout all of the trials, while the readout unit was kept outside of the refrigerator, in a tray 

taped to the refrigerator door. In this way, we were able to simultaneously measure both the 

internal refrigerator temperature and the external ambient temperature using the same device. By 

plotting the relationship between internal refrigerator and ambient temperature, we can determine 

the effect an increase in room temperature can have on vaccine storage conditions. 
 Page 18 of 41 

y = 0.091x + 2.3947

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

19.5 20.5 21.5 22.5 23.5 24.5 25.5

 Room Temperature, °C

Fridge Probe Temperature, °C

Fig. 8. Refrigerator temperature versus ambient temperature 

Figure 8 shows the results of one of the increased room temperature trials. This data was 

recorded over a period of 7.5 days. Each day, the room temperature control was increased by 

approximately 1 °C, allowing for adequate stabilization time of both the ambient and 

corresponding refrigerator temperatures. We also performed two shorter trials, in which the 

room temperature control was set to increase by either 3 °C or 5 °C, and the ambient and 

refrigerator sensors were monitored overnight. The data from the shorter trials was similar to 

that of the trial shown above. 

From Fig. 8, it appears that the ambient temperature and the corresponding internal refrigerator 

temperature are directly related. As the room temperature rises, so does the temperature of the 

vaccines stored inside the refrigerator. Extenuating factors, such as refrigerator load density, 

timing of a cooling cycle, and initial refrigerator temperature, are likely to complicate this 

equation beyond a simple linear relationship. However, it is helpful to examine a linear fit 

applied to the data to determine whether any generalizations can be made. 

Although the relationship between exterior and interior refrigerator temperature is inherently 

complex, we can extract simple and important observations from our brief investigation into this 

phenomenon. A linear fit applied to each of the data sets results in a slope of ~ 0.1 °C. This 

means that each time the room temperature rises one degree, we can expect the internal 

refrigerator temperature to increase approximately one-tenth of a degree. For the refrigerator 

temperature to rise a full degree, the room temperature would need to increase by 10 °C. 

As such, while room temperature fluctuations of 1 °C to 2 °C are not likely to cause significant 

danger to refrigerated vaccine storages, it is clear that some circumstances could cause a serious 

issue. For example, an air-conditioning outage during the summer time may result in a room 

heating up very quickly. During an actual AC outage, the ambient temperature may rise much 

higher than in our tests. If this type of event occurred on a weekend or another time when 

employees are not present to diagnose the problem, there is a strong possibility of internal 

refrigerator temperatures varying outside of acceptable vaccine conditions. 
 Page 19 of 41 

3.1.5. Effect of a Power Outage 

To observe what occurs following a power outage, we performed two trials in which the closed 

refrigerator was unplugged and monitored overnight. During these trials, the room temperature 

was stable to within 1 °C. In both trials, all of the thermometers recorded a fairly steep increase 

in temperature that slowed down slightly over time. All of the thermometers, including those 

attached to vaccine vials, exceeded the allowed maximum temperature in 5 h for the medium 

density (no bottles) trial, and in 9 h in the high density with bottles trial. Figure 9 below shows 

the temperature readings of all of the thermometers during one of the power off trials. The 

sudden drop in temperature following the peak corresponds to the point at which the refrigerator 

was plugged back in. 

-2

0

2

4

6

8

10

12

14

16

18

20

22

0:00 2:30 5:00 7:30 10:00 12:30 15:00 17:30 20:00 22:30 25:00

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top)

Fig. 9. Thermometer response during a power-outage event. 

The next graph (Fig. 10) shows the data collected starting from the time that the refrigerator was 

unplugged, at 14:15. The dashed red line indicates the 8 °C upper limit, and the blue dashed line 

shows the 2 °C minimum. 
 Page 20 of 41 

0

2

4

6

8

10

12

0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00

Time elapsed since power off, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top)

Fig. 10. Expanded view of thermometer response during a power-outage event. 

To determine the required response time to preserve vaccine viability during a power outage 

situation, we examined the amount of time elapsed from the point that the refrigerator was 

unplugged until the thermometers attached to vials exceeded 8 °C. This information is reported 

in Table 6 below. 

Table 6. Time after power-off until vial temperature exceeded 8 °C. 

High Density Mixed Medium Density Mixed

Name and Location With Bottles No Bottles

(hrs:mins) (hrs:mins)

vial 13 (in tray on upper shelf) 1:55 1:35

vial 11 (inside original packaging, middle shelf) 3:05 2:07

vial 10 (inside original packaging, middle shelf) 3:22 2:15

vial 20 (on top of box, mid-lower shelf) 4:16 2:42

vial 12 (inside cardboard box, middle shelf) 4:34 3:34

vial 6 (inside cardboard box, floor level) 8:37 4:47

We drew several conclusions from this data. The vials that exceeded the maximum temperature 

the most quickly were those that were not contained in boxes or packaging and were stored in the 

upper region of the refrigerator. This occurs for two obvious reasons. With the power off, the 

fans do not circulate air throughout the cavity, so the warmer air rises to the top of the 

refrigerator. Hence, vials stored in the top portion of the refrigerator are exposed to warmer air 

than those stored in the bottom. The vials contained in their original packaging or in cardboard 

boxes fared better than their “exposed” counterparts because of the additional layer of thermal 

insulation. In order to heat up a vial inside a container, thermal energy first must be conducted 
 Page 21 of 41 

through layers of cardboard. As such, to preserve vaccine efficacy in the event of a power 

outage, it is best to keep more sensitive medicines stored inside boxes and away from the upper 

region of the refrigerator. 

Table 6 shows the effect of storage method and location on the amount of time elapsed before 

vaccines exceeded the maximum temperature limit. Depending on the location and storage 

container for a particular vaccine, workers have between 1.5 h and 4.5 h before the vaccines 

warm above 8 °C to get the power restored or to move the vaccines to a new location. During 

the first trial, a vial stored inside a box on the floor maintained temperature for over 8 h. In spite 

of this, storing vaccines on the refrigerator floor is not recommended because the floor tends to 

be colder than the bulk of the refrigerator, and thus may keep vaccines below the allowed 

minimum temperature of 2 °C (more critical than that of the allowed maximum temperature of 

8 °C) during normal operation. 

The results of these trials further support the efficacy of using water bottles as a temperature 

ballast. Including water bottles in the refrigerator door provided an extra 20 min to 3 h 50 min 

before the vials exceeded the temperature limit, depending on their location. 

0

2

4

6

8

10

12

14

16

18

20

0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30

Time elapsed since power on, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top)

Fig. 11. Recovery after restoration of power for a freezerless refrigerator. 

In order to observe the time required for the refrigerator to return to equilibrium following a 

power outage, we continued recording data after the refrigerator was powered. Obviously, the 

length of the power outage and the resulting maximum temperature attained will affect the length 

of time required to get the refrigerator back down to its original baseline. 
 Page 22 of 41 

During the high density mixed load (including water bottles) trial, the refrigerator was left 

unplugged for a total of 15 h 31 min. During the medium density mixed load trial, the total 

power off time was 17 h 59 min. Because of this, the refrigerator reached a different maximum 

temperature in each trial, so the effect of loading density and water bottles on power outage 

recovery time cannot be evaluated based on these trials. The main point to note from the above 

data is that vials open to main refrigerator air flow returned to the specified temperature range 

the most quickly, while those contained in a thermally insulating package, such as a cardboard 

box, took the longest to recover to an allowed temperature. In both trials, the freezerless 

refrigerator took at least 6 h to completely re-equilibrate from ambient following the simulated 

power outage. 

3.1.6. General Stability of Vial Temperature and Suitability of Freezerless Refrigerator 

Model for Maintenance of Vaccine Storage Conditions 

 

The freezerless refrigerator model’s performance appears suitable for vaccine storage. Vaccine 

vial temperatures remained within the specified 2 °C to 8 °C range throughout all of the packing 

variation trials, as well as during the 15 second door opening trials. Drift of the refrigerator 

control thermometer appears to be negligible. 

3.1.7. Best and Worst Locations for Vial Storage 

Throughout the course of the closed-door freezerless refrigerator trials, vaccine vials remained 

within the desired temperature range regardless of their location in the refrigerator. However, 

based on the results of the door opening and power-off trials, it appears that some storage 

methods and locations within the refrigerator may be better for vaccine storage than others. In 

general, vials stored inside cardboard shipping boxes or in their original cardboard packaging 

experienced the least temperature disruption during normal, periodic door-opening operation. 

Storage in plastic trays also provided suitable thermal insulation. The vials that recorded the 

largest temperature fluctuations were kept open to the air and placed directly on refrigerator 

shelves or on top of boxes, close to the front of the refrigerator. This comes as no surprise, since 

this type of storage method provides very little thermal insulation from the surge of warm, 

ambient air that enters the refrigerator each time the door is opened. 

During the simulated power outage trials, the vials placed in the bottom portion of the 

refrigerator were the last to exceed the allowed maximum temperature. Again, storage inside 

cardboard boxes provided the greatest level of thermal insulation, followed by storage inside the 

original (thinner) cardboard packaging. During normal operation, fans inside the refrigerator 

distribute warmer and cooler air to maintain a fairly uniform temperature distribution throughout 

the interior space. Once a power outage begins, pockets of warmer and cooler air begin to 

separate, as the cooler air settles around the floor of the refrigerator and the warm air rises 

towards the top. As a result, vials kept on the upper shelves and not contained in boxes 

experienced the fastest temperature increases. 
 Page 23 of 41 

3.1.8. Best and Worst Methods of Monitoring Vial Temperature 

In this study, we used several different temperature sensing devices in various locations 

throughout the refrigerator in order to determine the best way to monitor actual vial temperature. 

While attaching a thermocouple thermometer to a vial is a very accurate way to determine 

vaccine temperature, this is not a feasible strategy for clinics or storage facilitities. To determine 

the suitability of different vial temperature monitoring schemes, that is, the best and worst 

locations and monitoring devices for this purpose, we can compare the temperature readings of 

the vial-attached thermocouples to the readings of the other thermometers. 
 Page 24 of 41 

Sensors Attached to Vaccine Vials

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

6 (vial - floor)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

20 (vial - low)

Sensors Attached to Walls

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

Sensors in Glycol Filled Bottles

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

LD (glycol - top)

Sensors Hanging in Air

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

5 (air)

7 (air)

8 (air)

9 (air - top)

Sensors on Shelves and in Trays

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

14 (inside tray)

15 (back of tray)

LA (floor)

LC (mid)

Medium Density Trays - All Sensors

0

1

2

3

4

5

6

7

0:00 0:03 0:06 0:09 0:12 0:15

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top)

Fig. 12. Response of sensors with different mounting methods. 
 Page 25 of 41 

The graphs in Fig. 12 show a 15 minute period of data collected during the trial with a medium 

density packing of trays, with water bottles. This section of data is representative of the standard 

operating mode of the refrigerator observed throughout the course of our study. The first five 

graphs each contain data for thermometers placed in different types of locations (see Fig. 2 for 

thermometer locations). In this case the thermometers are classified by their location inside the 

refrigerator, whether they are attached to a vaccine vial, a wall, kept inside a glycol filled bottle, 

hanging in the air, or placed on a shelf or in a tray. The sixth graph shows the same data excerpt, 

but contains the readings for all of the thermometers. Using these graphs, we can determine 

which thermometer locations and types gave readings that most closely matched the actual vial 

temperatures. 

Best Locations for Temperature Sensors 

 The thermocouples contained inside glycol filled bottles and placed in the central or upper 

portion of the refrigerator matched the vaccine temperatures closely (18, 19). During the 

door-opening and power-off trials, these thermometers displayed temperature increases 

similar in magnitude and rate to those recorded by the vial thermometers. 

 Data logger D (LD), with its probe contained in a glycol filled bottle placed in the upper 

region of the refrigerator, also matched the vial thermometer data fairly well. In general, the 

Data logger D gave temperature readings 0.5 °C to 1 °C higher than the bulk of the vial 

thermometers. However, this logger was placed one refrigerator shelf higher than the vials, 

so this discrepancy may be related to location rather than logger performance. 

 As a general rule, the closer the thermometer is placed to the actual location where vaccines 

are stored, the closer its readings will be to actual vaccine temperatures. 

Mediocre Locations for Temperature Sensors 

 Thermometers placed on main refrigerator shelves, in the air, or inside of a tray in the areas 

where vaccines were stored produced data reasonably similar to that of the vial thermometers 

most of the time (5, 7, 8, 14, LC). 

 However, the thermocouple attached to a tray (14) read consistently colder values than the 

vial thermometers. 

 The thermometers hanging in the air space close to the vials gave readings that were fairly 

representative of vial temperature during the closed door testing, but showed much greater 

sensitivity to door opening and power outages than the vial thermometers. If we were to rely 

upon one of these thermometers for vaccine temperature monitoring, we would likely 

experience multiple “false alarms” that the temperature had exceeded 8 °C. 

 In addition to the sensitivity issue noted above, data logger C (LC) recorded temperatures an 

average of 1 °C higher than the vial temperatures. Since LC was placed on the same shelf as 

several vials, this is most likely a result of the logger’s resolution, which is much lower than 

that of the thermocouples, or an issue with its calibration. Even so, as long as the refrigerator 

is kept in the middle of the 2 °C to 8 °C range, logger LC placed near the vials could be 

sufficient for vaccine temperature monitoring. 
 Page 26 of 41 

Unacceptable Locations for Temperature Sensors 

 Placing a thermometer on the floor of the refrigerator will result in readings colder than the 

actual vial temperatures. Thermocouple 17, inside a glycol filled bottle, should have been 

able to closely match vial thermometers due to its similarity to a vial (liquid inside a glass 

bottle) and high temperature resolution. However, it consistently recorded temperatures 

almost a degree colder than the vials.

 Data logger A (LA) also suffered from being placed on the floor. For the most part, 

temperature fluctuations recorded by data logger A had the same magnitude as those 

experienced by the vials. However, the logger readings were consistently cooler than the 

vials. Data logger A should be tested further using a more centralized placement in the 

refrigerator. Under this condition, it is likely provide very accurate monitoring of vial 

temperature.

 Thermometers attached to upper refrigerator walls or anywhere in the top airspace of the 

refrigerator performed extremely poorly (1, 2, 4, 9). These thermometers were subject to 

dramatic fluctuations in response to normal refrigerator temperature control and defrost 

cycles as well as during our simulation trials. Thermometers placed in this area are right next 

to the cooling unit, so they pick up a major temperature change each time the refrigerator 

starts or stops pumping out cold air. 

 Refrigerator walls generally should not be used as an indicator of main cavity temperature. 

The thermometer attached to the back of a tray (15) did not perform well either, possibly due 

to its close proximity to the wall. 

3.1.9. Defrost Cycle Issues 

Throughout the course of the freezerless refrigerator measurements, we noted the occurrence of 

periodic temperature spikes due to the refrigerator’s defrost cycle. These temperature spikes 

generally appeared once every 2 to 3 days. As evidenced by the graph below (Fig. 13), which 

shows a standard freezerless refrigerator model defrost cycle, vaccine vial temperatures were not 

greatly affected by these periodic temperature increases. During this particular cycle, none of the 

vial-attached thermometers recorded temperatures above the 8 °C temperature limit. 

Occasionally, the temperature spike would be larger, and some of the vials would briefly exceed 

8 °C. In this case, the time that the vials were out of the allowed temperature range was 

generally between 5 min and 15 min. 
 Page 27 of 41 

-1

2

5

8

11

14

17

0:00 0:15 0:30 0:45 1:00

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LA (floor)

LC (mid)

LD (glycol - top)

Fig. 13. Temperature spike due to a defrost cycle. 

However, depending on the placement of thermometers used to monitor vaccine temperatures, 

the incidence of a defrost cycle could cause a VFC worker to incorrectly surmise that vaccine 

temperatures strayed outside of both the allowed maximum and minimum temperature bounds. 

Referring again to the graph, some of the thermometers attached to walls or hanging in the air 

recorded quite dramatic temperature spikes followed by a significant drop well below the 2 °C 

limit. If one of these thermometers were used to ascertain vial temperatures, then employees 

may improperly adjust the refrigerator set-point temperature and dispose of vaccines needlessly. 

As a result, care must be taken in determining the best placement for temperature monitoring 

devices in order to most accurately determine vial temperature during events such as refrigerator 

defrost cycles. 
 Page 28 of 41 

3.2. Dormitory Style Refrigerator 

3.2.1. Effect of loading density 

In trials performed using the dorm-style refrigerator, vaccine storage density (see Fig. 14) had a 

noticeable effect on the refrigerator’s ability to maintain consistent and uniform vaccine storage 

temperatures. 

Fig. 14. Three packing densities tested with the dormitory style refrigerator. 

In particular, higher density packing increased the range of location-specific temperature 

variation. This is reflected in the Fig. 15 below, as the vertical spread of the data is larger for the 

trials with a high density loading scheme than in the lower density trials. Since the dormitorystyle refrigerator does not have a fan system, a high packing density most likely limits air flow, 

creating “pockets” of hot and cold air. 
 Page 29 of 41 

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

lowtrays

medtrays

medtrays bottles

medboxes bottles

medtrays bottles

highboxes bottles

lowmixed bottles

medmixed bottles

highmixed bottles

highmixed bottles lights out

highmixed bottles opendoor

medmixed

medmixed opendoor

lowtrays

lowtrays opendoor

Temperature, °C

1 - top LH wall

2 - mid LH wall

3 - low LH wall

4 - top back wall

5 - hanging LH mid

6 - vial (LR)

6- vial (floor)

7 - hanging RH mid

8 - hanging low

9 - fridge probe

10 - inside box

11 - inside box

12 - vial (LH floor)

13 - vial (RH LR)

14 - inside LR tray

15 - back LR tray

17 - glycol (floor)

18 - glycol (LR)

19 - glycol (UR)

20 - vial (floor)

LD (glycol LH UR)

LB (floor)

LC (LR)

LA (floor)

Fig. 15. Average temperature of dormitory-style refrigerator for all sensors, as a function of 

loading pattern. Labels on the abscissa correspond to entries in the measurement matrix of 

Table 1. 

After 15 days of the total 42 day duration of the measurements on this refrigerator, the 

refrigerator ceased to maintain the required 2 °C to 8 °C temperature. In Fig. 15 above, starting 

from the 9th trial (high density mixed, with bottles), the majority of the thermometers recorded 

average temperatures below the allowable minimum temperature of 2 °C. The control settings of 

the refrigerator were left unchanged throughout the duration of the study, so these results are 

indicative of a problem with the refrigerator model’s temperature control—the set point appears 

to drift over time. The dorm-style refrigerator does not appear capable of maintaining the narrow 

temperature range required for any reasonable length of time, nor does it maintain a consistent 

temperature profile thorough the interior of the refrigerator. Such a lack of temperature control 

will require periodic adjustment to the set-point temperature in an effort to maintain the required 

2 °C to 8 °C temperature range; this will result in both unacceptable thermal excursions and a 

decrease in operational efficiency. These observations alone demonstrate that the dormitory-style 

refrigerator model is not suited to vaccine storage. 

The lack of air circulating fans makes the formation of cold and warm air pockets extremely 

likely, especially in the case of high density loading patterns. Because the dormitory-style 
 Page 30 of 41 

refrigerator is small to begin with, it is likely to be over-stuffed at times, rendering it even more 

unreliable. As a result, based on the results of our study, we do not recommend that the 

dormitory-style refrigerator be used for vaccine storage under any circumstance. 

3.2.2. Effect of packing style 

The magnitude of overall set point drift and unpredictable temperature fluctuations was so large 

that any effect caused by the different packing styles (trays, boxes or a mix of both, as shown in 

Fig. 16) was indeterminable. 

Fig. 16. Packing styles tested with the dormitory-style refrigerator. 

3.2.3. Effect of door opening and closing—normal use simulation 

Prior to the start of the door opening and closing trials, the majority of the thermometers 

(including all of those attached to vials) recorded temperatures well below the allowed minimum, 

to as low as –5.16 °C (recorded by vial 12 at the start of the low density trays run). Since the 

dorm-style refrigerator was unable to perform to the specified requirements even under idealized, 

closed door operation, its performance during periodic door opening and closing is a somewhat 

moot point. We note, however, that the majority of the thermometers actually recorded an 

overall average temperature decrease in response to the door opening. 

Figure 17 below shows the temperature data from a door-opening trial in which a high density 

mixed (with bottles) packing scheme was used. As seen in the graph, many of the thermometers 

recorded a brief temperature spike each time the door was opened. As expected, the most 

dramatic temperature increases were recorded by the thermometers hanging in the air. 
 Page 31 of 41 

-8

-6

-4

-2

0

2

4

6

8

10

12

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45 2:00 2:15 2:30 2:45

Duration of measurement, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - floor)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - floor)

LD (glycol - top)

LA (floor)

LC (mid)

Fig. 17. Door-opening trial for the dormitory-style refrigerator. 

In spite of the brief temperature spikes caused by periodically opening the door, the majority of 

the thermometers actually recorded an overall average temperature decrease in response to the 

door opening. This was probably caused by the refrigerator cooling unit working harder to 

counterbalance the influx of warmer ambient air. During the last door-opening trial, in which a 

low-density packing scheme was used, all of the thermometers showed a marked average 

temperature decrease, recording average temperatures between 0.74 °C to 3.19 °C lower than 

during the closed-door trial with the same packing scheme. In this case, even if the refrigerator 

set point was within the specified range at the start of the trial, the temperature decrease 

associated with door opening could have easily caused the vaccine temperatures to drop below 

the allowed minimum by the end of the trial. This adds to the evidence that dormitory-style 

refrigerators are not acceptable for vaccine storage. 

Only one door-ajar trial (without use of bottles) was performed on the dormitory-style 

refrigerator, so no comment can be made regarding the bottles’ efficacy in this case. At the time 

the refrigerator was opened, all of the thermometers read temperatures much lower than the 

allowed minimum (vial thermometers ranged from –2.8 °C to –8.1 °C). Even so, all of the vial 

temperatures exceeded the 8 °C maximum before the door was closed again, as shown in the 

Table 7 below. 
 Page 32 of 41 

Table 7. Time after opening the door until vial temperatures exceeded 8 °C. 

Low Density Trays

Name and Location No Bottles

vial 20 (open to air, floor level) 18 mins

vial 12 (open to air, on top of box on middle shelf) 26 mins

vial 13 (open to air, in tray on middle shelf) 29 mins

vial 11 (inside original packing, middle shelf) 31 mins

vial 10 (inside original packing, middle shelf) 33 mins

vial 6 (open to air, in tray on floor level) 37 mins

The refrigerator door was closed after being left ajar 1 h 15 min; the vials took approximately 3 h 

20 min to return to their steady-state temperature. 

3.2.4. Effect of Increasing Room Temperature 

The increasing room temperature trials for the dormitory-style refrigerator were performed in the 

same manner as with the freezerless model. As before, an increase in ambient temperature was 

positively correlated with increasing refrigerator temperature. Fig. 18 below shows a plot of 

internal refrigerator temperature versus ambient, as recorded by the two data logger D probes. 

The graph shows the data from the high-density mixed-packing (with bottles) trial; the other 

trial, with a medium-density mixed load, produced similar results. In this case, we can use a 

linear fit to approximate the relationship between internal refrigerator and ambient temperatures.

y = 0.2869x - 5.9402

0.0

0.5

1.0

1.5

2.0

2.5

3.0

23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0

Ambient Sensor Temperature, °C

Fridge Probe Temperature, °C

d

Fig. 18. Refrigerator temperature versus ambient temperature. 

The slope of the linear fit applied to the above data is ~ 0.3 °C/°C. A similar graph for the 

medium-density load trial produced a slope of ~0.3 °C/°C. This means that for every 1 °C 

increase in room temperature, the internal refrigerator temperature rises by approximately 0.3 °C. 

This is almost triple the effect observed in the freezerless refrigerator trials. Clearly, the dormstyle refrigerator is much more susceptible to even minor room temperature fluctuations than the 

freezerless model. An ambient temperature increase of only 3 °C could cause the interior 
 Page 33 of 41 

temperature of the dorm-style refrigerator to rise by 1 °C. This is of significant concern when 

stored vaccines must be kept within a 2 °C to 8 °C temperature range. The results of the 

increasing room temperature trials further demonstrate the deficiencies of this particular 

refrigerator model as a vaccine storage unit.

3.2.5. Effect of a Power Outage 

We performed two power-outage simulation trials using the dormitory-style refrigerator, in 

which the closed refrigerator was unplugged and monitored overnight. In both trials, most of the 

thermometers recorded initial temperatures well outside of the allowed range before the 

refrigerator was even powered off. In the first trial, a high-density mixed (with bottles) load, 14 

of the 22 thermometers, including all but one of the vial-attached thermometers, had initial 

temperature readings below 2 °C. For example, the reference thermocouple (#18, in a glycol 

bottle in the middle of the refrigerator) read 1.1 °C at the start of the test. In the second trial, a 

medium-density mixed (no bottles) load, all of the thermometers had initial temperatures well 

below 2 °C. In this case, the same reference thermocouple had a starting temperature of 

–1.22 °C. 

The internal refrigerator rose fairly quickly after the initial shut off of power, and then gradually 

leveled off at ambient room temperature. The following two graphs (Fig. 19 and 20) show the 

initial temperature rise for each trial, from the time that the refrigerator was unplugged until all 

of the thermometer readings exceeded the 8 °C limit, shown on the graphs as a red dashed line. 

The blue dashed line indicates the 2 °C lower limit. 

-4

-2

0

2

4

6

8

10

12

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00

Time elapsed since power off, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LD (glycol - top)

LA (floor)

LC (mid)

Fig. 19. Expanded view of thermometer response during a power-outage event for a high-density 

loading with bottles. 
 Page 34 of 41 

-6

-4

-2

0

2

4

6

8

10

0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00

Time elapsed since power off, h:min

Temperature, °C

1 (top wall)

2 (mid wall)

3 (bottom wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (in box)

11 (in box)

12 (vial - mid)

13 (vial - mid)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - low)

LD (glycol - top)

LA (floor)

LC (mid)

Fig. 20. Expanded view of thermometer response during a power-outage event for mediumdensity loading without bottles. 

The dormitory-style refrigerator has already repeatedly proved itself to be unsuitable for vaccine 

storage, even under idealized, closed door unadjusted operation. As a result, attempting to 

protect vaccines stored in a dormitory-style refrigerator during a power outage is problematic. 

Even so, we can make some interesting observations on the length of time elapsed from the start 

of the outage until the vaccine vials exceeded the upper temperature limit. This information is 

summarized in the Table 8 below. 

Table 8. Time after power outage until vial temperature exceeded 8 °C. 

High Density Mixed Medium Density Mixed

Name and Location With Bottles No Bottles

(hrs:mins) (hrs:mins)

vial 10 (inside original packing, middle shelf) 4:42 3:05

vial 11 (inside original packaging, middle shelf) 4:47 3:14

vial 12 (open to air, on top of box on middle shelf) 5:20 3:02

vial 13 (open to air, in tray on middle shelf) 5:30 3:24

vial 20 (open to air, floor level) 6:02 3:48

vial 6 (open to air, in tray on floor level) 7:16 3:13

It is immediately obvious that the refrigerator exceeded the 8 °C temperature limit much more 

quickly during the second trial (with a medium-density, mixed load and no water bottles) than 

during the first (high-density load, with bottles). (See Fig. 21.) This occurred despite the fact 

that the median starting temperature was approximately 4 °C colder than that of the second trial. 

This is further proof of the effectiveness of water bottles stored in the refrigerator door in 

resisting temperature change. 
 Page 35 of 41 

 High Density Load, with Bottles Medium Density Load, No Bottles 

Fig. 21. Refrigerator loading at different densities and with and without water bottles. 

3.2.6. General Stability of Vial Temperature and Suitability of Dormitory-Style 

Refrigerator Model for Maintenance of Vaccine Storage Conditions 

Throughout the tests using the dormitory-style refrigerator, vial temperatures rarely remained 

within the specified range. During the first two trials (low and medium density trays), 

thermometers 10 and 11 remained within 2 °C to 8 °C temperature range. However, both of 

these thermometers were attached to vials inside the same original package, so this cannot be 

taken as representative of vials stored throughout the refrigerator space. Prior to the fifth trial 

(medium-density trays with bottles), additional thermometers were attached to vials spread out 

around the refrigerator. From this point on in the study, during every single trial, at least one or 

more monitored vial strayed outside the allowed temperature range. From these results, it is 

clear that the dormitory-style refrigerator can not be relied on to maintain vaccine storage 

temperatures, regardless of the packing density or storage containers used. 

3.2.7. Best and Worst Locations for Vial Storage 

The dormitory-style refrigerator’s performance was consistently unacceptable, regardless of 

vaccine storage location within the refrigerator. As a result, it is not possible to recommend a 

“good” storage area. Anywhere on or near the floor, or near to the cooling and freezer unit in the 

top right corner appear gave particularly unstable results. However, since the refrigerator space 

is quite small, and lacks any type of air circulation system, virtually any storage spot will be 

relatively close to one of these “bad areas.” Because of this, the dormitory-style refrigerator is 

NOT recommended for vaccine storage under any circumstance. 

4. DATA LOGGER EVALUATION 

Throughout the course of trials performed on both refrigerators, we tested the performance of 

several electronic temperature loggers along with the thermocouples. The data loggers tested, 
 Page 36 of 41 

labeled LA, LB, LC, and LD, are described in Table 2. We wished to evaluate the data loggers’ 

effectiveness at monitoring vial temperature. 

At present, many VFC clinics use digital display, analog, or liquid in glass thermometers to 

monitor the temperature of their vaccine storage refrigerators. They are only required to check 

and record the temperature readings of these thermometers twice each working day. Based on 

the results of our study, a number of factors, including the frequency of refrigerator door 

opening, refrigerator defrost cycles, room temperature increases, and power outages, could cause 

an employee to obtain a false representation of the actual refrigerator and/or vial temperature 

based on this methodology. For example, an overnight power or AC outage could cause the 

refrigerator and corresponding vaccine temperatures to rise outside of the viable range. If the 

outage is corrected before the next morning, the refrigerator may return to its cooled equilibrium 

state by the time employees come to work and check their temperature monitoring devices. In 

this case, they would fail to detect that the vaccine vials exceeded the allowed temperature range 

overnight, rendering them potentially unusable. This type of situation could pose a serious 

danger to clinic patients, since they could be unknowingly administered an ineffective vaccine. 

In the same light, other conditions could cause employees to record a “false positive” 

temperature, and discard vaccines unnecessarily. Depending on a thermometer’s accuracy, its 

placement inside the refrigerator, and the time that it is checked, its reading may or may not 

correspond to actual vaccine vial temperature. Some thermometers are more sensitive to 

temperature fluctuations than the fluid-filled vaccine vials, which have a larger thermal mass. If 

an employee happened to read one of these thermometers during the refrigerator’s standard 

defrost cycle, which produces a brief air-temperature spike, he or she might determine that the 

vaccines had exceeded the allowed temperature limit. In our experiments, we found that the 

actual vial temperatures were not greatly affected by the defrost cycle, but that some of the other 

thermometers indicated significant temperature increases. Because of these issues, we believe it 

is necessary for vaccine storage refrigerators to have in place continuous data loggers to obtain 

truly representative, reliable readings of actual vaccine temperatures. 

In addition to providing continuous temperature data, the widespread use of data loggers in VFC 

clinics would reduce the overall workload involved in refrigerator temperature monitoring. At 

present, VFC workers must manually record refrigerator temperature two times a day. The VFC 

coordinator is then responsible for reviewing hand-written temperature logs to determine if any 

lapses occurred. Electronic data loggers connect to a computer via USB and can quickly 

download data through easy-to-use software interfaces. Logger data for multiple days are 

automatically summarized into spreadsheets and graphs in a manner of minutes, relieving the 

VFC coordinator of all data entry and analysis work. As a result, we believe that the use of data 

loggers for vaccine temperature monitoring would not only improve the reliability this process, 

but also make it significantly more efficient. 

Since the data from the dormitory-style refrigerator tests showed dramatic temperature 

fluctuations and a large location-specific temperature spread due to the poor performance of the 

refrigerator, we will only consider data from the freezerless refrigerator trials in our evaluation of 

the data loggers. While some of the data loggers performed better than others, all of the loggers 

gave temperature readings within their specified resolution (0.5 °C or 1 °C) of the readings from 
 Page 37 of 41 

the vial-attached thermometers during the majority of the freezerless refrigerator trials. During 

the door opening trials, the data loggers showed very small temperature increases in response to 

the door opening, similar to the vials themselves. 

When the freezerless refrigerator underwent a defrost cycle, we observed that data logger D and 

data logger A logger readings matched those of nearby vial thermometers. Data logger C 

showed a larger temperature spike than any of the vials, exceeding the 8 °C limit. This may have 

been due to its placement directly on a shelf rather than inside a box or a tray, or may have 

simply been a function of the sensitivity of the device. 

In the power outage simulation trials, all of the data loggers exceeded the 8 °C maximum 30 min 

to 2 h 30 min before the vials placed nearest to them exceeded the limit. The largest 

discrepancies were associated with the vials stored inside of cardboard boxes, which took the 

longest to warm up to room temperature following the start of the power outage. In general, if 

the data loggers are stored in the same fashion as the vaccine vials, whether in trays or inside 

cardboard boxes, and in the same part of the refrigerator, they will most likely give readings 

within their resolution (0.5 °C or 1 °C) of the actual vial temperatures. In the case of a power 

outage, unless the problem can be remedied very quickly, the vaccines are likely to become 

unusable. As a result, the function of the data loggers in this case is more to demonstrate the 

occurrence of an outage rather than to predict the exact temperatures of the vials at a given time. 

While the data logger A readings appeared to be fairly representative of nearby vial temperatures 

throughout the various trials, we noticed that the logger took a very long time to equilibrate to 

refrigerator temperatures after it was removed to room temperature air during the data 

downloading process. Because of data logger A’s heavy-duty metal structure, it has a large 

thermal mass and takes a significant amount of time to equilibrate to the surrounding air 

temperature. In our trials, the cool-down time for data logger A was as much as 3 or 4 h. During 

this time period, the logger is not very useful as a vial temperature monitoring device. Data 

logger C, which had a lighter plastic exterior, still required approximately an hour of 

equilibration time once it was taken from room temperature and placed inside the refrigerator. 

Data logger D, which has a temperature sensor inside the main unit as well as an external probe 

attached via cable, eliminates the need to wait for thermal equilibrium after replacing a room 

temperature logger back into a refrigerator. In our tests, the external probe was inserted into a 

glycol filled bottle placed inside the refrigerator, while the main device unit was kept outside of 

the refrigerator. To collect data from the logger, we simply detached the main unit from the 

cable, leaving the external probe inside the refrigerator. The main unit was then connected to the 

USB data collection cradle in order to download the logger readings. This allowed us to keep the 

external probe at refrigerator temperatures throughout the study, eliminating the need to wait for 

device equilibration following each course of data collection. The data logger D design has an 

additional advantage in that main unit of the device, if kept outside of the refrigerator, can be 

used to simultaneously monitor room temperature. The detachable-probe data logger design is 

the most user-friendly and for convenience displays the internal refrigerator temperature and 

external room temperature outside of the refrigerator. 
 Page 38 of 41 

Overall, the data loggers’ performance seems to be sufficient for monitoring vaccine 

temperatures. The most important factor for temperature monitoring accuracy is the placement 

of the device inside the refrigerator. Just as vaccines should not be stored on the refrigerator 

floor, neither should the data loggers used to monitor vaccine temperatures. This is because the 

floor tends to be much cooler than the rest of the refrigerator’s interior. During our testing, data 

logger A was placed on the refrigerator floor, and as a result, it tended to record temperatures 

0.5 °C to 1 °C cooler than the thermometers attached to vaccines stored in the center of the 

refrigerator. A configuration like that of data logger D, in which an external temperature probe 

was kept inside of a glycol-filled bottle, is likely to give the most representative picture of the 

actual vaccine vial temperatures. Regardless of the type of data logger chosen, it should be 

stored in the same manner as the vaccines in order obtain the most accurate readings. Simply 

placing a data logger directly on a refrigerator shelf is not likely to give an accurate 

representation of vial temperatures. If the vaccine vials are stored inside trays or boxes, the data 

loggers should be stored in the same way.
 Page 39 of 41 

5. SUMMARY 

Table 9 below summarizes the results obtained in this study. 

Table 9. Comparison of refrigerator performance in response to tested criteria. 

COMPARISON OF REFRIGERATOR PERFORMANCE 

IN RESPONSE TO TESTED CRITERIA

Criteria Freezerless Refrigerator Dorm-Style Refrigerator

Loading density  No impact on performance 

 Noticeable impact on 

performance due to lack of air 

circulation 

 High-density loading patterns 

increased location-specific 

temperature variation 

Packing style 

(Trays, Boxes or Mixed)  No impact on performance  Indeterminable 

Opening/ closing 

refrigerator door 

 Vial temperatures showed small 

increases in response to door 

opening, but remained in 2 °C to 

8 °C range throughout trial 

 Thermometers in air / attached to 

walls exceeded 8 °C 

 Keeping water bottles in door 

reduced temperature change. 

Without use of bottles, temperature 

increase up to 1.2 °C higher 

 Most thermometers recorded 

brief increases in response to 

door opening, but showed 

overall temperature decrease 

 If drifting refrigerator set point 

had been corrected prior to start 

of trial, vial temperatures still 

may have dropped below 

allowable range 

Door left ajar 

 All vial temperatures exceeded 8 °C 

within 1 min to 49 min, depending 

on location and storage method 

 Vials stored inside boxes or original 

packaging stayed within allowed 

range up to 48 min longer than 

uncontained vial 

 Use of water bottle ballast provided 

an extra 2 min to 15 min before 

vaccines exceeded limit 

 All vial temperatures exceeded 

8 °C within 18 min to 37 min, 

depending on location and 

storage method 

Increasing room 

temperature 

 Room and refrigerator temperature 

directly related 

 For every 1 °C increase in room 

temperature, refrigerator 

temperature rises approximately 0.1 

°C 

 Small room temperature 

fluctuations will not greatly impact 

refrigerator performance 

 Room and refrigerator 

temperature directly related 

 For every 1 °C increase in room 

temperature, refrigerator 

temperature rises 

approximately 0.3 °C 

 Small room temperature 

fluctuations pose a greater 

threat to vaccines stored in this 

refrigerator 
 Page 40 of 41 

Criteria Freezerless Refrigerator Dorm-Style Refrigerator 

Power outage 

 Depending on vial location and 

storage container, it takes 1.5 to 

4.5 h for vials to exceed 8 °C after 

power off 

 Water bottle ballast provided an 

extra 20 min to 3 h 50 min before 

vaccines exceeded limit 

 Vials stored in boxes or original 

packaging, stored away from the 

upper region of refrigerator had 

slowest temperature increase 

 Requires at least 6 h to fully reequilibrate following an outage 

 

 Power outages exacerbate already 

poor refrigerator temperature 

control 

 Water bottles provided 

significant thermal insulation—in 

trial with water bottles included, 

vials took an extra 1 hr 40 min to 

4 h 14 min to exceed 8 °C, even 

though initial temperature was 4 

°C colder during no bottles trial 

General stability of vial 

temperature 

 Good 

 Vaccine vials remained within 

specified temperature range 

during regular operation, 

regardless of packing density or 

type 

 Extremely poor 

 Vaccine vials were rarely kept 

between 2 °C to 8 °C 

 In nearly every trial, one or more 

monitored vials strayed outside 

of this range 

Refrigerator temperature 

control drift  Negligible 

 Severe temperature control drift 

 Within 2 weeks of use, median 

temperature drifted ~ 4 °C lower 

than original set point 

 Would require constant 

adjustment to maintain specified 

range 

Defrost cycle 

 Defrost cycle runs every 2-3 days 

 Vials occasionally exceeded 8 °C 

for less than 15 min 

 Thermometers placed in air / near 

walls recorded dramatic 

temperature spike followed by a 

drop below 2 °C 

 No defrost cycle 

 Refrigerator interior quickly 

becomes encased in frost and ice 
 Page 41 of 41 

ACKNOWLEDGMENTS 

We thank the Centers for Disease Control and Prevention for providing financial support for this 

project. Additional thanks to Dawn Cross, who assisted with the Vaccines for Children clinic 

tours. 

REFERENCES 

1. Day, G., Rossetto, B., and Ho, J., Testing of Purpose-built Refrigerating Vaccine Storage 

Cabinets, Test Research Report E175VC-01, Marrickville, New South Wales, 2009. 

2. Page, S. L., Earnest, A., Birden, H., Deaker, R., and Clark, C., “Improving vaccination cold 

chain in the general practice setting,” in Australian Family Physician 37, No. 10 (2008). 

3. Proceedings of the National Vaccine Storage Workshop, edited by A. Langley, and S. Grant., 

Queensland Health, Brisbane, 2004. 

4. “Performance characteristics of domestic fridges for vaccine storage,” E2E IT Solutions Pty. 

Ltd. Available from: http://www.temptrack.com.au/documents/Fridge Performance Report 

(Rev A).pdf 

5. Vaccine Storage and Handling Toolkit, Centers for Disease Control. Available from: 

http://www2a.cdc.gov/vaccines/ed/shtoolkit/