The lab I interned in after my bachelor's did a plague (Yersinia pestis) challenge study for some collaborators who were testing an edible plague vaccine produced in tomato plants. The authors confirmed that the vaccines were immunogenic but sent them to my PI for the actual challenge study. But the mice were so old and decrepit by the time they got to us (I suspect they were the actual individual mice from this paper) that they died anyway - they were REALLY ancient-looking mice. So we don't know if it would have been protective or not in younger animals.
I think what you are refering to was an oral vaccine, which uses plant as delivery mechanism. Its a kind of recombinant vaccine? It is possible either for big protein seed as well. Thats my opinion.