Vaccines: Preparing the Immune System In Advance For Invasion

The Theory Behind Vaccination
Canine infectious diseases are caused by organisms which gain access to the body, multiply, and in the process of their life cycles cause severe and in some cases irreparable damage to the cells which make up organs and tissues of the body. Even in animals with normal immune function, invasion and damage can proceed at a rate faster than the immune system's ability to destroy the invader. In cases where organ function is severely compromised, the dog may succumb to the disease before the immune system can eradicate the infection, or in cases where the infection is eliminated, death or debilitation may still occur as a result of irreparable cellular damage.

Based on the knowledge that the immune system responds much more rapidly if it encounters an invading organism that it has already battled and defeated, the theory that introducing just enough antigen into the body to illicit an immune response without causing disease would protect the body from contracting the disease at a later time gave rise to the procedure of vaccinating. Therefore, "vaccination," also known as "active immunization" refers to the procedure whereby administration of an antigen results in protective immunity to the disease associated with that antigen.

Types of Vaccines

Killed vaccines. Killed vaccines are composed of "inactivated" microorganisms which cause a particular infectious disease. Because these microorganisms are dead they are unable to replicate once introduced into the dog's body and therefore are incapable of producing disease. However, their presence will induce an immune response. Therefore, in terms of some adverse reactions, killed vaccines are considered to pose fewer risks. In terms of protective immunity, however, killed vaccines produce weak immune responses and provide a shorter duration of protective immunity. In many cases, killed vaccines must be administered in large or frequent antigenic doses to induce a sufficient immune response to yield protection in the event of disease exposure.

Modified live vaccines. Modified live vaccines are composed of "attenuated" microorganisms; that is, these microorganisms associated with a particular disease are altered so that they do not cause infection in most dogs, but they are still capable of replicating and inducing a protective immune response. Because these microorganisms are still capable of replicating and spreading throughout the body like an infectious agent, they elicit a stronger protective immunity of longer duration. As such, however, a higher frequency of adverse reactions is associated with use of modified live vaccines (discussed below) and therefore, not all dogs are good candidates for immunization with modified live vaccines.

Subunit vaccines. Subunit vaccines are composed not of the whole microorganism but only a component of the microorganism which will produce an immune response. Therefore, subunit vaccines are similar to killed vaccines in that they are not infectious and therefore, also present a low risk for adverse reactions. However, as with killed vaccines, subunit vaccines cannot replicate and, therefore, do not provide strong protective immunity for long periods of time. Because of these factors as well as the higher cost for production, subunit vaccines are used less frequently than modified live and killed vaccines.