[refurb]

The second option. Stability testing is done quite early with any drug candidate and is a requirement for FDA approval. There is zero chance that Pfizer didn't test their vaccine at higher temperatures.

Although Moderna's and Pfzier's vaccines are both mRNA technologies, they aren't identical in either the mRNA itself or the lipid nanoparticle used to deliver the mRNA. That's why they have different stability profiles.