The mRna molecule and it’s host solution was designed and tested to be more stable at higher temperatures. It was a molecular engineering and pharmaceutical tour de force....that said, it does have issues...as in short term of development...troubling adverse reactions...no permanent effectiveness...unknown long term effects. What new facts will we discover down the road?
I thought only the J&J vaccine was designed for higher temperatures. It’s a little different from the other mRNA vaccines in that it doesn’t directly inject mRNA. Instead it injects a virus carrying DNA which then gets converted by the cell into mRNA. Not sure why this approach avoided the super low temp requirement but IIRC it did.