A common question we are asked is why pets of different sizes need different doses of medication(s), but the volume of their vaccine(s) are the same. It is a very good question!

When doctors or veterinarians are prescribing or administering a drug, their goal is to achieve a desired concentration of the drug in a targeted tissue. To get there, most drugs will be transported in the bloodstream and diluted across all of the tissues in the body – not just the desired tissue. The drug(s) will then be removed from the body over time – usually by the liver and/or kidneys. Most of the body is water, so if you can imagine color dyeing one end of a pool by placing dye in another end of the pool while the pool filter is running…you can start to visualize how, as your pool starts to get bigger, you’re going to need a lot more dye to get the job done. This is why drug doses will change, depending on the weight or size of the patient receiving it.

Vaccines work by exposing the body to a set amount of pathogen proteins, or something that looks close enough that the immune system cannot tell the difference. These proteins do not get distributed through the body the way drugs do. Instead, the immune system reacts locally, learns about that disease, then sends trained cells and antibodies throughout the body. The dose of vaccine to be administered, therefore, is not determined by the patient’s body size, but rather by the amount of protein required to mount an effective immune system response, which does not change by patient size.

To summarize: drugs are systemic, so their dose is dependent on body size; vaccines are local, so the dose remains independent of body size.  There is one consistency between drugs and vaccines: if the appropriate dose is not given, it will likely not work!