Also, I'm not sure what part of the peanut butter is the more important for the allergy. If it is a protein, perhaps a mRNA shot can make it better/worse, because the mRNA shot make some of your cell produce a protein.
If the allergenic substance in peanut butter is not a protein, I think it would be more difficult to make a mRNA shot that is good/bad.
So all this would do is introduce a lot of protein directly into serum. If someone was allergic, that would cause a reaction, and if not, it's not clear what it would do. It may not be a good idea for it to suddenly appear in the body without having been exposed through the GI tract.
[Disclaimer: In case it's not obvious, I'm not a medical doctor. https://xkcd.com/938/ ]