Usually, if your vet orders you to give a medication to your dog or cat, your vet will measure out an appropriate dose according to its weight. Then, why is giving a vaccine one size fits all? Why does a five-pound Chihuahua receive the same dose of a vaccine as a 50-pound golden retriever?
Quite simply, a little goes a long way when it comes to vaccines. The way that vaccines work is by triggering the animal's immune system to kick into gear to fight a harmful substance that it is being exposed to. This is not something measured by weight. Dogs and cats of all sizes get the same dose, but much larger animals like horses and elephants will get more since those animals' sizes are much larger by several orders of magnitude
The reason weight matters with antibiotics is that the antibiotic must reach an effective level in the blood and tissue to fight infection. The bigger the animal, the more area to cover.
Animal Dosing Sizes
In most cases, a vaccine dose is 1 milliliter and is given via a syringe subcutaneously, intramuscularly, or in the nostrils or mouth (bordatella vaccine for kennel cough).
Vaccines are given to puppies and kittens at least six to eight weeks of age and adult dogs and cats of all sizes. Rabies is given later, typically between four and six months of age for dogs and cats.
Much larger animal species get larger vaccine doses. For example, dogs and cats may get a 1 ml dose, horses and cattle will get 2 ml, and elephants can get two to four ml.
Any animal of any size can have a reaction to a vaccine at any time. Small dogs (under approximately 20lbs) are slightly more likely to have a reaction to vaccines, especially when multiple vaccines are given together. For this reason, it is usually recommended that very young or small bodyweight animals, should get vaccines spread out. For example, you might want to give the distemper vaccination, wait for two to four weeks, then give the rabies vaccination. This is currently the case in veterinary practice. Normal puppy and kitten core vaccination schedules are usually given at three- to four-week intervals until 16 to 18 weeks of age. You can speak to your veterinarian about your pet's specific vaccination protocol.
Does a Half Vaccine Dose Work?
In a word, no. A half vaccine dose is not reliable. The protection—namely, the immune response that your pet's body needed to have—is unlikely to have been enough to protect it against future exposure to that substance. And, along those same lines, the vaccine manufacturer claims the protection level of a vaccine is not adequate if not given according to established guidelines.
Multiple Doses and Booster Shots
Animals (as well as humans) get several dosages of the same vaccine over time. After initial immunization, a booster injection is a re-exposure to the substance. A booster is intended to increase immunity against that antigen back to protective levels after memory against that antigen has declined through time. The frequency of revaccination needed is dependent on the type of vaccine administered.
For example, dogs and cats are first vaccinated for rabies between four and six months of age. They need a booster one year from that date. They are then vaccinated every three years, although some states still require annual rabies vaccinations for dogs and/or cats.