The immune response works as follows: Strange new protein in your body -> immunity

mRNA is the blueprint to a protein. You can get the mRNA into your body directly (super cool novel tech) or using a carrier virus that has the blueprint as DNA (eg. a modified chimpanzee cold in astrazeneca's covid vaccine). Here, the body first has to turn DNA into mRNA.

The researchers show that the latter approach gives rise to errors in prodcing proteins, errors that can cause severe side effects.