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 5-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 (bordatella vaccine for kennel cough).
Vaccines are given to puppies and kittens after 6 weeks of age and adult dogs and cats of all sizes. Rabies is given later, typically between 4 and 6 months of age for dogs and cats.
Much larger animals 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 2 to 4 ml.
Any animal of any size can have a reaction to a vaccine at any time. It might stand to reason that smaller animals might get more reactions to vaccines, but this is not the case.
However, it is recommended that very young or small body weight animals, should get vaccines spread out to allow the body's immune system to not be overwhelmed with mounting a response. For example, you might want to give the distemper vaccination, wait for 1 to 2 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 3- to 4-week intervals until 16 to 18 weeks of age. You can speak to your veterinarian for 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—may not have been enough to protect it against a future exposure to that substance. That administered vaccine is now a question mark. 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.
For example, dogs and cats are first vaccinated for rabies between 4 and 6 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.