Data-Logger Thermometers for Continuous Temperature Monitoring of Refrigerated Vaccines

Background: Complete and accurate records of vaccine temperature history are vital to preserving drug potency and patient safety, yet many VFC providers do not use continuous temperature monitoring systems in their vaccine refrigerators. There is a need for improved, measurement-based guidelines for vaccine temperature monitoring, including device selection, accuracy requirements, proper use, and methods for maintaining measurement traceability and validation.

Objectives:  We evaluated the performance of seven digital data logger models for candidates as refrigerated vaccine temperature monitors, based on the following criteria:

• Out-of-box performance and compliance with manufacturer accuracy specifications over the range of 0 °C to 10 °C
• Measurement stability over time (6 - 18 months of continuous use)
• Proper use in a vaccine storage refrigerator so that measurements reflect liquid vaccine temperatures
• Practical methods for end-user validation and establishing metrological traceability

Methods:  Data loggers were tested using ice melting point checks and by comparison to calibrated thermocouples to characterize performance over 0 °C to 10 °C. We also monitored logger performance in a study designed to replicate the range of vaccine storage and environmental conditions encountered at provider offices.  

Results:  For effective temperature monitoring, data logger setup must mimic the conditions and physical properties of stored vaccines. A data logger with an external probe kept in glycol-filled bottle effectively accomplishes this with minimal effort or expense. All loggers tested using this setup consistently performed within manufacturer accuracy specifications at both the ice point and in refrigerator trials. End-users may successfully validate performance of these logger types in the field using a simple ice point check. By contrast, loggers featuring sensors designed to record air temperature proved unacceptable for use as vaccine temperature monitors.

Conclusions:  The results of this study provide the CDC with the much-needed scientific basis to disseminate improved vaccine temperature monitoring policies.