How does the immune system work?
Here is a basic overview of how your immune system works. You get infected with some virus. The virus has antigens. Your body has antibodies. The antibodies attack the virus and “remember” the antigens. This is how your immune system builds immunity: it gets exposed to a new disease and it creates antibodies to those antigens. When we give vaccines, the vaccines have the antigens to the disease we are trying to vaccinate the animal for. So the immune system is tricked into thinking it’s being infected and it develops antibodies to match the vaccine antigens.
In order for any vaccine to be approved, it has to go through testing. Rigorous testing. The rabies virus is such a deadly and communicable disease, there are very strict laws about the rabies vaccine. Every dog must be vaccinated with a rabies vaccine at regular intervals. In order to be granted a one year interval, researchers must take a group of animals (lets say dogs for this instance). They isolate the dogs and give them a rabies vaccine. The dogs must remain completely isolated from here on out. Exactly one year later, the dogs are given a live rabies virus. If the dogs survive the infection, the rabies vaccine is said to be “certified” for a one year interval. But in order to get a two year certification, the same study must be done only the dogs have to stay isolated for two years. As you can see, this would get prohibitively expensive and cruel to the animals to keep them completely isolated for three, four, or five years. This is why no rabies vaccine is certified for that long of an interval: not because the vaccine might not be effective passed one year but because in order to confirm how long it actually works for, an untold number of animals would have to be but through massive amounts of pain and suffering to prove the point.