Monitoring of Vaccine Refrigerators
NIST performed a study on the effects of opening and closing vaccine refrigerator doors on the temperature measured inside the storage compartment.
As we have examined in previous blog posts, it is essential to the efficacy of the vaccine that it is stored at stable temperatures, between 2°C – 8°C. In the absence of proper monitoring of early warning systems and protections, there can be a substantial waste of vaccines, diminished potency and the need for revaccination.
NIST’s thermodynamic Meteorology Group has examined and evaluated the CDC recommended practices for cold-chain management systems and methods of vaccine storage used in the vaccines for children (VFC) programs.
The evaluation uncovered several issues in the cold-chain distrbution and storage of vaccines under the VFC program. Through this study NIST was able to identify suiable commercial refrigerators for vaccine storage as well as evaluating the impact of the operating conditions on the performance of the refrigerator. Further than this, NIST determined the proper placement of thermometers so previously undetected temperature excursions could be identified. This has all contributed to improvements in cold-chain management and vaccine storage practices.
NIST began the project in 2009 after CDC approached NIST to conduct a study on the storage and temperature monitoring inefficiencies that were resulting in large losses from discarded, potentially ineffective vaccines. The VFC program distributes approximately $4 billion USD of vaccines per year to protect children from low-income families through over 44,000 VFC providers/clinics throughout the United States.
An analysis published in the journal Vaccine estimated that anywhere between 14-35% of vaccines have been subjected to accidental freezing, and accounts for the largest source of vaccine losses. There may be additional temperature excursions that are undocumented, meaning ineffective vaccines could unknowingly have also been passed onto patients.
The below image shows the recommended location of temperature data loggers in your vaccine refrigerator. It should be on the middle shelf, inside a medicine bin and inside a sealed jar of glycol to act as a buffer.
NIST has also studied the optimal amount of glycol buffer to use with temperature data loggers. These buffers mimic the vaccine fluid temperature and help ensure accuracy in the collected data. When the refrigerator door is opened or closed a temperature sensor exposed to the air would potentially be affected showing an excursion.
Air temperature changes much more rapidly due to the quick movement of molecules within the air. When the door is opened these warmer air molecules enter the refrigerated compartment. A temperature data logger exposed to the air would react to this change. It does not mean however that the temperature of the vaccines has reacted in the same way. The heat transfer laws mean that the vaccine liquid would not instantly take on the temperature of its surroundings. How quickly the vaccine obtains thermal equilibrium depends on many factors. By placing the temperature data logger in a jar of glycol it acts as a buffer to simulate the rate of change of temperature in the vaccine fluid
The below video gives an overview of how to comply with CDC vaccination storage requirements.
Even when the door is not opened, if the refrigerator is performing its defrost cycle the evaporator coil behind the refrigerator will heat up, enough to melt any ice that has formed inside. Due to the heat generated during this time, a thermometer that is not in a glycol buffer will be affected and show a potential false alarm for a temperature excursion. The reality is however that the vaccine fluid has maintained it’s stability between 2-8°C. This could result in the disposal of a perfectly viable vaccines.
CDC guidelines for vaccine storage require a detachable sensor that is placed in a bottle filled with glycol. Glycol is the preferred medium because it has a low freezing temperature which allows for more accurate temperature measurement.
The below diagram illustrates the required dimensions of the bottle and the proper spacings between the sensor and the glass jar.
The diameter of the bottle should be a minimum of 4x the diameter of the probe. The tip of the probe should be at least 1-2cm above the bottom of the jar, with a minimum immersion depth of 10x the probe diameter. The lid should be sealed to ensure that air cannot get inside.