This is kind of what I was thinking about. Or even something less sophisticated; I vaguely remember a story about some plants that were cross-bred or spliced o

This is kind of what I was thinking about. Or even something less sophisticated; I vaguely remember a story about some plants that were cross-bred or spliced onto tobacco, because the resulting nicotine in the leaves kept plant-eating insects away (and I believe there was some biological reason the nicotine couldn't cross into the edible parts, but it's been forever so I don't really remember). Frankly, I don't even know that "vaccine" is a useful categorization for this. Vaccines, as a category, are defined by their outcome and intent, not their constituent parts or methodology. A properly refrigerated mRNA injection for COVID is a vaccine. If you heat that same vial up to 100C, it's not a vaccine anymore, it's just some junk in a vial. At the same time, a properly-refrigerated mRNA shot and a deactivated virus shot are both vaccines, even though their method of action and production share little in common.