Vaccinations control or prevent numerous human and animal diseases. For a vaccine to be effective, the immune response must enhance the aspect of immunity that will be effective against specific pathogens. For example, vaccines against extracellular bacteria should induce IgG to opsonize the bacteria for phagocytosis, activate complement, and neutralize toxins. Other immune factors are necessary for other pathogens.

Modified live vaccines (MLVs) mimic virulent organisms, providing the necessary immune response. However, MLVs have serious potential disadvantages and are undesirable for some diseases. Killed vaccines may not provide the necessary signals to induce protective immunity. Adjuvants provide artificial signals to the immune system. Research is necessary to determine the appropriate adjuvant needed to invoke the type of immune response. Most adjuvants are chemicals, microbial components, or mammalian proteins. A single adjuvant may have more than one mechanism of action, but the exact mechanisms are still somewhat speculative. Adverse effects from the interaction of adjuvants and antigens include fever, arthritis, uveitis, anorexia, soreness, and lethargy. In dogs, rabies or distemper combination vaccines are most often associated with local reactions. Local reactions in cats have been linked to the development of vaccine-associated sarcomas. They have been linked to rabies, FeLV, and other vaccines as well as injections of nonvaccine products. No specific brands of vaccines seem to be associated with sarcoma. New adjuvants are being developed, representing diverse chemical compounds, different mechanisms of action, and different potential adverse effects. Reducing adverse effects is one of the goals of adjuvant research.

Preparation of the original manuscript was partially supported by a grant from Fort Dodge Animal Health, Fort Dodge, IA.

Adjuvants in veterinary vaccines: modes of action and adverse effects. Spickler AR, Roth JA. J VET INTERN MED 17: 273-281, 2003.