At the same time, freezing in vials is inherently hard to control. At commercial scale, tens of thousands of vials are typically packed together in a pallet and frozen slowly in a cold storage room. In addition to the inherent stochasticity of ice nucleation, spatial differences in heat transfer across the pallet imply that freezing depends on vial position. Furthermore, the thermal evolution of a vial's neighborhood also shapes its own behavior due to thermal interactions resulting from the employed dense packing. To complicate things further, the means for process monitoring are limited; in the case of the Janssen COVID-19 vaccine, a few thermocouples were inserted into a pallet to monitor its thermal evolution and to identify the endpoint of the process.