https://blogs.sciencemag.org/pipeline/archives/2020/04/15/co... is a post from April of last year detailing the different types of vaccines, which does mention some of the issues mRNA vaccines have had in the past. I think there's a fuller discussion of mRNA somewhere on that blog, but the gist of it is "we've been trying this for about a decade and we might finally have all the kinks worked out."