What to know
Each facility should have proper temperature monitoring equipment that is set up correctly, maintained appropriately, and repaired as needed.
Temperature Monitoring Devices
Every vaccine storage unit must have a temperature monitoring device (TMD). An accurate temperature history that reflects actual vaccine temperatures is critical for ensuring vaccine quality and potency. In addition, investing in a reliable device helps avoid the financial hardships of replacing vaccines that have lost their potency due to storage at out-of-range temperatures.
Digital Data Logger
A digital data logger (DDL) is a specific type of TMD that has the capability for continuous monitoring and provides the most accurate storage unit temperature information, including details on how long a unit has been operating outside the recommended temperature range (referred to as a "temperature excursion"). Unlike a simple minimum/maximum thermometer, which only shows the coldest and warmest temperatures reached in a unit, a DDL provides detailed information on all temperatures recorded at preset intervals.
Many DDLs use a buffered temperature probe1, a device that provides the most accurate measurement of vaccine temperatures. In contrast, standard thermometers typically measure air temperature. Probes permanently embedded in a buffer are acceptable if the temperature monitoring system for the entire unit can be calibration-tested. Since certain DDLs cannot monitor ultra-cold temperatures, it's crucial to use a probe intended for ultra-cold environments.
Temperature data from a DDL can either be downloaded to a computer using special software or retrieved from a website. The software or website may also allow providers to set the frequency of temperature readings. Reviewing DDL data is critical for ensuring vaccine potency, so it is important to decide whether independent software or a website program works best for facility.
☑ 🖍 Use DDLs for routine, on-site vaccine storage; during vaccine transport; and in temporary, mobile, off-site, satellite, and community vaccination clinics.
☑ 🖍 Place a DDL or other appropriate TMD in each vaccine storage unit and each transport unit.
☑ 🖍 Have at least one backup DDL or TMD in case a primary device breaks, malfunctions, or requires calibration.
TMD Placement
It is important to position the TMD correctly so that it accurately reflects the temperature of the vaccines. Refer to the owner's manual for detailed instructions for TMD placement.
If using a DDL in a storage unit:
- Place the buffered probe of the DDL in the center of the unit with the vaccines surrounding it. A device placed near the unit's walls, floor, vent, ceiling, or door may indicate temperatures that are colder or warmer than the actual vaccine temperature. Certain purpose-built units may recommend alternate locations or have designated areas for probe placement. Consult the owner's manual for detailed instructions on proper TMD placement in these units.
- Place the DDL's active digital display outside the unit so temperatures can be read without opening the door and disturbing the probe.
Temperature Monitoring Device Limitations
Some types of TMDs have significant limitations and should not be used to measure temperatures in vaccine storage units. These devices can be difficult to read and only display the temperature at the moment they are checked, which means they may fail to detect temperature excursions.
CDC does not recommend the following TMDs:
- Alcohol or mercury thermometers, even if placed in a fluid-filled, biosafe, liquid vial
- Bimetal stem TMDs
- TMDs used for food
- Chart recorders
- Infrared TMDs
- TMDs that do not have a current and valid Certificate of Calibration Testing
Important: Some devices sold in hardware and appliance stores are designed to monitor temperatures for household food storage. They are not calibrated or accurate enough to ensure vaccines are stored within correct temperature ranges. Using these devices can pose a significant risk of damaging vaccines.
Calibration Testing of TMDs
Calibration testing ensures that TMDs provide accurate readings in line with nationally accepted standards.
☑ Calibration testing should be performed every 2 to 3 years, or according to the manufacturer's recommended schedule.
Over time, TMDs may experience a "drift," affecting their accuracy. Testing verifies that the device's accuracy continues to meet nationally accepted standards.
Mishandling a TMD can also affect its accuracy. If a TMD is dropped, hit against the side of a storage unit, or is potentially damaged in any way, its accuracy should be checked against another calibrated TMD. If there is any question about accuracy, the device should be replaced or sent for calibration testing.
Certificate of Calibration Testing
☑ 🖍 Each DDL or appropriate TMD should have a current and valid Certificate of Calibration Testing.
Keep Certificates of Calibration easily accessible for reference whenever needed.
To determine if an appropriate entity issued a Certificate of Calibration Testing or Report of Calibration, check to see if the certificate indicates one or more of the following:
- Conforms to International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 17025 international standards for calibration testing and traceability3
- Performed by a laboratory accredited by International Laboratory Accreditation Cooperation (ILAC) Mutual Recognition Arrangement (MRA) signatory body
- Traceable to the standards maintained by the National Institute of Standards and Technology (NIST)
- Meets specifications and testing requirements for the American Society for Testing and Materials (ASTM) Standard E2877 Tolerance Class F or higher
- Refers to another acceptable accuracy validation method, such as comparison to other traceable reference standards or tests at thermometric fixed points
☑ 🖍 Each Certificate of Calibration Testing should include:
- Model/device name or number
- Serial number
- Date of calibration (report or issue date)
- Recommended uncertainty2 of ±0.5°C (±1°F) or less
- Confirmation that the instrument passed testing (or instrument is in tolerance)
- Some vendors that perform calibration testing only issue certificates for units that pass. In these instances, the certificate may not explicitly state "passed." If there is no indication of a passing result on the certificate, a statement from the vendor outlining their certification process may be kept on file as an alternative.
If calibration testing indicates that a TMD is not accurate within ±0.5°C (±1°F), the TMD must be replaced.
Power Supply
Even with appropriate equipment and temperature monitoring practices in place, power disruption can occur and result in destruction of the entire vaccine supply. Precautions should always be taken to protect the storage unit's power supply.
☑ Plug in only one storage unit per electrical outlet to avoid a fire hazard and power source overload, which could cause the power to shut off.
☑ Use a safety-lock plug or an outlet cover to prevent the unit from being unplugged.
☑ 🖍 Post "DO NOT UNPLUG" warning signs at electrical outlets and storage units to help ensure staff, custodians, electricians, and others do not unplug units.
☑ Label fuses and circuit breakers (e.g., with "DO NOT STOP POWER signage) to alert people not to turn off power to a storage unit.
☑ Use caution when using power outlets that can be tripped or switched off. Avoid using
- Built-in circuit switches (may have reset buttons).
- Outlets that a wall switch can activate.
- Multi-outlet power strips.
If built-in circuit switches, Uninterruptible Power Supply (UPS) unit, or power strip surge protection must be used, make sure the device is rated to carry the maximum current as specified by the storage unit's manufacturer. Additionally, consider how the device manages when the power is restored. Whether the device automatically restarts and allows the equipment to run, or must be manually switched on, should be considered and represented in Emergency Plans and SOPs. Contact the unit manufacturer for any additional questions or guidance regarding circuit switches, power strips, UPS, or surge protection.
If the entire storage unit is affected by a temperature excursion because of a power supply issue, unit malfunction, or other issue, refer to the facility's emergency SOPs.
Regular Maintenance of Temperature Monitoring Devices
Storage units and TMDs require regular maintenance to ensure proper operation and extend the useful life of the equipment. Check the manufacturer's product information for cleaning instructions and recommended maintenance schedules. Document maintenance tasks and repairs, as indicated in the facility's routine storage and handling plan and SOPs.
☑ Conduct routine maintenance on all vaccine storage units and related equipment to ensure they function at maximum efficiency.
- Check seals and door hinges.
- Clean coils and other components per the manufacturer's directions.
- Manual-defrost freezers should be defrosted when the frost exceeds either 1 cm or the manufacturer's suggested limit. Follow the manufacturer's instructions. While defrosting, store vaccines temporarily in another unit with appropriate freezer temperatures.
- Clean the interior of each unit as needed to discourage bacterial and fungal growth. Do so quickly to minimize the risk of a temperature excursion.
- 🖍 Test any backup generator quarterly and have it serviced annually.
Repeated Alarm Alerts
If the temperature alarm sounds repeatedly, do not disconnect it until the cause is identified and addressed. Do basic checks of the unit door, power supply, and thermostat settings. If the alarm continues to trigger or the temperature remains outside the range, transfer the vaccines to a backup unit as directed by the facility's SOPs. A repair technician should check the equipment to determine the need for repair or replacement.
- Battery changes may affect temperature accuracy and may warrant checking against a known, calibrated TMD. Check with the device's manufacturer for specific information on battery changes.
- Buffered Temperature Probe: A temperature probe designed to prevent false excursions by protecting the thermometer from sudden changes in temperature that can occur when opening a refrigerator door. A probe is “buffered” by immersing it in a vial filled with liquid (e.g., glycol, ethanol, glycerin), loose media (e.g., sand, glass beads), or a solid block of material (e.g., Teflon®, aluminum).
- Uncertainty:The quantification of doubt about the measurement result.
- Traceability:An unbroken chain of measurements and associated uncertainties.