Vaccination is an important tool in veterinary medicine, preventing disease and reducing virus circulation for many important viral pathogens. In small animal medicine, vaccines for a multitude of agents are available, and the decisions regarding what vaccines to use and how often to use them can be difficult to determine. This discussion will cover the various general types of vaccines available, and summary of recommended guidelines for selection.

Vaccines are designed to enhance the specific immune response to a particular pathogen in order to prevent disease upon exposure and natural infection. This is accomplished through inoculation with all or part of an organism. The development of a vaccine must take into consideration the type of immunity needed, the role of memory cells, safety, cost, stability, and ease of administration. Vaccine types can be divided into two basic categories: noninfectious and infectious.

Noninfectious Vaccines

Noninfectious vaccines have certain advantages. There is no chance of reversion to virulence since the virus is "dead". These vaccines are generally stable, and safe; thus they can be used in debilitated or pregnant animals with minimal risk. However, because there is no active replication, the response stimulated is primarily humoral, with little if any cell-mediated component. The duration of immunity tends to be shorter than that of infectious vaccines, and multiple boosters are often required. Because of the addition of adjuvant, and the relatively high antigenic mass, adverse reactions may be more common, including hypersensitivity reactions and granuloma formation. Examples in small animal medicine include most rabies vaccines, and most bacterins.

Infectious Vaccines

Infectious, or live vaccines contain viable, replicating organisms. These organisms have been modified in some way that reduces or eliminates their disease-producing capabilities while maintaining their antigenicity. The majority of infectious vaccines are attenuated whole viruses, containing the relevant pathogen which has undergone multiple mutations designed to abolish its pathogenicity. Newer vaccines use recombinant technology, including vector viruses or DNA plasmids that express immunogenic viral proteins. Recombinant vaccines are currently available, and most use a canarypoxvirus as the vector for various virus proteins such as the coat proteins of canine distemper virus and rabies virus.

Infectious vaccines have a lower antigenic mass, and require no adjuvant. Because of their replicative ability, these vaccines stimulate both humoral and cell-mediated immunity, and generally fewer boosters are required to induce and maintain protection. There is some, albeit low risk for reversion to virulence. Contamination of live vaccines with adventitious agents has rarely occurred leading to serious consequences. Generally, infectious vaccines are of lower stability and require maintenance of the cold chain. Many are freeze-dried, and once rehydrated, have a short period of viability. Because the agents are infectious, there is some risk for immunosuppressed animals, and significant risk for the fetus in pregnant animals.

Vaccine Selection

In considering which vaccines to use, one must consider not only the epidemiology and threat of a particular pathogen, but also the chances of the individual animal to exposure, and the properties of the vaccines available, including safety, efficacy and cost. Many vaccines are multivalent, necessitating only a single inoculation to induce immunity to multiple agents. While no consensus exists, guidelines are available from various professional organizations, including the AVMA, ACVIM, and AAFP.