There is some research and experimentation into genetically engineering vaccines within plants, like bananas and tomatoes. Vaccines today are very expensive to produce and sometimes have poor shelf lives, but by putting vaccines in foods those problems may be fixed.
Most likely these vaccines would be "grown" in a controlled environment where cross-pollination and adverse effects on other species would not be possible. But, considering what would be the widespread use and disposal of these foods, there is potential that the gene could enter the domestic food supply. Ingestion in large quantities and ingestion by certain people with health conditions may be very harmful. There also is the primary issue of the effectiveness of the modified foods as a vaccine.
It seems unlikely (and unsafe) at this point, but someday instead of a painful shot we may be immunized by a tasty banana.