NIST Case Study of Dual-Zone Unit for Simultaneous Refrigerated and Frozen Vaccine Storage

Background: While the majority of vaccines administered through CDC programs require refrigerated storage, Varicella-containing vaccines must be stored at freezing temperatures (between -50 °C and -15 °C) to maintain drug efficacy. As a result, providers often choose to purchase dual-zone refrigerator/freezer units to fulfill all of their vaccine storage needs with one unit. During the course of the NIST refrigerated vaccine storage and handling study, we received several reports of dual-zone refrigerator compartments becoming excessively cold after adjusting the freezer compartment temperature. We performed a dual-zone case study aimed at addressing some of these questions about synchronized refrigerator and freezer operation.

Objectives: To determine whether adjusting the freezer temperature control dial of our dual-zone unit had an effect on the temperature inside the refrigerator compartment.

Methods: We installed 19 thermocouples and 6 data logger thermometers throughout the refrigerator and freezer compartments of our dual-zone unit, which featured independent refrigerator and freezer temperature control dials. While keeping the refrigerator control dial constant, we adjusted the freezer temperature dial to different set points and recorded the subsequent temperature effects in both cavities.

Results: Refrigerator compartment temperature was largely unaffected by changing the freezer temperature. Some refrigerator locations experienced very small temperature changes, but remained well within the prescribed range for vaccine storage (2 °C to 8 °C). However, the freezer temperatures measured during our study raised significant questions as to whether this type of unit is even acceptable for frozen vaccine storage (between -50 °C and -15 °C).

Conclusions: To date, the suitability of dual-zone freezer compartments for frozen vaccine storage has not been thoroughly investigated. Our results highlight the need for a more in-depth study, while underscoring the importance of widespread continuous temperature monitoring to verify that the correct storage temperatures are consistently maintained in both refrigerated and frozen vaccine storage compartments.