When most people think of vaccines, they think of preventing infections from bacteria and viruses. Giardia are protozoans, and are eukaryotes like animals, plants, and fungi, although they are not part of any of those kingdoms. Bacteria are in a separate domain by themselves, and viruses are classified differently because they aren't considered to be quite alive. My understanding is that a vaccine for a protozoan would work more like a heartworm preventative than a vaccine for a bacterial or viral disease.