Propylene Glycol is a carrier liquid for the vaccine.
It is a common ingredient in ice cream (to make the ice cream not freeze too hard) and was a major component in vaping.

Propylene Glycol has a freeze / melting temperature of -74 deg.F. I can see holding the vaccines in frozen state down to maybe -80 degrees F.
Anything colder doesn't make much sense.

You late night warriors are on it. I saw on hear the claim of keeping the vaccines cold -90(days ago). This has let my mind go on and on to why the fuck that's necessary. I'm not a virologist; however working in the lab that I did...every sample and anything to keep alive were stored in -80 C freezers. Nothing was removed unless active research was going on. I had to find a couple of articles over the temp to verify tonight.

I'm sure someone has a reason, but none of this seems normal.

Read that the mRNA is fragile and the Glycol stabilizes it.

If the mRNA strands are proteins, or amino acid strands, the longer they are “kept warmer”, the more the strands denature.

The Propylene Glycol helps ensure that ice crystals don’t form and fracture the mRNA.

This has bothered me too, just from simple chemistry. They seem pretty arbitrary about just how cold it needs to be and for how long.