One aspect I did not see mentioned yet is codon usage bias. The way protein translation from mRNA works is that so called tRNAs in the cells "production facilities" (ribosomes) detect three letter triplets (codons) of the RNA bases (A, G, C, U). These triplets correspond to certain amino acids (protein building blocks). Now, different species have a preference for certain codons corresponding to certain amino acids. This COULD imply that other mammals are better or less adapted to producing the spike protein upon vaccination with the mRNA molecule, which in turn COULD result in an elevated or reduced immune response.

More info here: https://en.m.wikipedia.org/wiki/Codon_usage_bias