> As I understand it, the reason an HIV vaccine has been so difficult to create is that HIV mutates so easily.
Nope. There are exactly two species of HIV since at least the '80s. HIV-1 and HIV-2. HIV however can trick the immune system into creating non-neutralizing antibodies, that can't fully prevent the progression of the infection.
If you run a large pharma company you probably already make a lot of money from long-term HIV/AIDS treatment. From the CEO's perspective developing a HIV vaccine may shrink the antiretroviral market if at-risk groups vaccinate themselves. Therefore you prioritize more lucrative projects.
HIV vaccine makes sense if:
1. You can hope for a general mandate of your product, or:
2. You are a small pharma company looking to make a name for yourselves.
Regarding the benefit of mRNA vaccines over non-mRNA vaccines for HIV, I don't know. I have yet to learn if there is significant medical benefit to mRNA other than reduced cost and TTM.
AFAIK mRNA does not enable producing new types of proteins, only this time they are produced by your body, right?