Used pvieito's algorithm and tweaked the GC ratio, thanks to practicalswift for coming up with the idea to optimize for the Japanese wild mouse. Currently at 91.65% match to the original vaccine sequence.
[1] Reverse Engineering Source Code of the Biontech Pfizer Vaccine
[2] A prefusion SARS-CoV-2 spike RNA vaccine is highly immunogenic and prevents lung infection in non-human primates
[3] Kazusa Codon Usage Database
[4] NCBI Taxonomy Browser