system
Given the following post, respond as if you're commenting on Reddit:
human
Why did they opt for an mRNA COVID vaccine as opposed to using said mRNA to generate the viral antigens and inject those instead?

I'd figure the viral antigens themselves would be a lot more stable than mRNA and maybe not need to be stored at such extremely cold temperatures. 

Since everybody is getting the same mRNA and thus generating the exact same viral antigens, why not just produce the antigens in situ (or in vivo with COVID-infectable animals), purify the viral antigens, and ship those as the COVID vaccine?
gpt
The immune system responds to a foreign protein in the context of self. It doesn't just respond to free floating stuff. It has to be internalized by a cell and presented on the surface of the cell to be recognized by the immune system. Manufacturing mRNA is much easier and cheaper than manufacturing a complex protein. Also, some proteins require post-translational modification such as the addition of sugar molecules or changes in the three dimensional conformation of the molecule. Letting the cell do this naturally is more accurate than trying to do it in a bioreactor. Further, there's a much smaller contaminant profile that has to be cleaned up if you're not having to remove partial protein fragments, etc.