It appears simple, but a whole lot of work went in to producing that string even pte-COVID. Some of it is generic in the sense that it might apply to any mRNA vaccine. Some is quite specific:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5584442/

There’s also (IIRC, no citation right now) prior work suggesting that coronavirus vaccines against the spike are likely to be effective and that vaccines against the N protein might be counterproductive.