It is because the mRNA vaccines must be kept at deep, deep freeze temperatures or even cryogenic temperatures. Polyethylene glycol is added to prevent it from freezing solid.
That was my initial suspicion, but it’s nonsensical when dealing with temperatures of -94ºF (Pfizer) and -4ºF (Moderna) and small volumes in vials.
In short, mRNA is so unstable (currently) that it’s frozen solid to thaw for use and uses PEG-2000 nanoparticles to both help protect the mRNA and assist the mRNA in cell penetration.
Isn’t that in radiator coolant?!?!