Vaccines use the principle of antigens to stimulate the immune system to protect against specific disease. An antigen is a foreign substance, typically a protein that induces an immune response in the body, such as the production of antibodies. Antigens are specific to each type of organism. For example, antigens for Eastern encephalomyelitis virus are different than those for Western encephalomyelitis virus. Once a horse’s immune system recognizes a specific antigen, memory within immune cells--so called the anamnestic response--facilitates a rapid response by the immune cells to create antibodies to combat the foreign antigenic protein.
Whether the vaccine given is made with a killed organism or a modified live (weakened) organism, the antigenic dose given to the horse is too small to create an actual infection or illness, but significant enough to mimic infection that stimulates development of antibodies to the foreign proteins (antigens).
To enhance a horse’s response to antigens within a vaccine, often the manufacturer includes an adjuvant--as for example, aluminum hydroxide. An adjuvant not only is able to boost the immune response and extend the duration of immunity, but it also can minimize the amount of foreign antigenic material necessary to include within the vaccine.
Without periodic boosters, the amount of antigens will wane over time. The more antibodies that are present, the better the immune response if the horse is exposed to the causative agent. Research has determined the antibody load that is necessary to ward off disease and this informs the manufacturer about recommended booster frequency. In most cases of equine vaccines, boosters are recommended annually or twice yearly to achieve optimum immune protection.