Vaccines have proven to be a major scientific advancement for people and animals for over a century. Vaccination is the most efficient, practical and cost effective means of controlling infectious diseases via prophylaxis.  The enormity of the benefit from vaccines is hard to comprehend and is one of the biggest reasons, if not the biggest reason we as a society currently enjoy our relative good state of health. Vaccination has been responsible for the eradication of small pox across the globe; the elimination of hog cholera and brucellosis from North America; and the control of diseases such as foot and mouth disease, pseudorabies, rabies, anthrax and rinderpest would not have been possible without the use of effective vaccines.

The general principle behind the use of vaccines is to introduce a modified and safe version of a given pathogen into an animal host which induces an immune response that will be protective to the animal in the future if and when it encounters a natural exposure to this pathogen.  There are two basic methods by which an animal may be made immune to an infectious disease.  One method is passive immunity which produces immediate but temporary resistance by transferring antibodies from a resistant animal to a susceptible animal.  Examples of this type of immunity include colostrum from the dam to the newborn and the use of tetanus and Clostridium perfringens C & D antitoxins.  The second method of immunization is called active immunization which involves administering antigens (foreign substance-vaccine) to an animal so that it responds by developing its own protective immune response.  Re-immunization or exposure to infection will result in a secondary immune response.  The disadvantage of active immunization is that protection is not conferred immediately.  The advantage of active immunization is that once it is established, it is long lasting and capable of restimulation.

Passive immunization requires that antibodies be produced in a donor animal by active immunization and that these antibodies are then given to susceptible animals in order to confer immediate but short lasting protection.  Serum containing these antibodies, (sometimes called serum antibodies, antiserums or antitoxins) may be produced against a wide variety of pathogens.  The most important role of antiserums is in the protection against toxigenic organisms such as Clostridium tetani or Clostridium perfringens. Antiserums made in this way are commonly produced in young horses by a series of immunizing injections.  The toxins of these clostridia are proteins that can be denatured and made nontoxic by treatment with formaldehyde.  This type of vaccine is known as a toxoid.  Donor horses are first given toxoids, but once antibodies are produced, subsequent injections of purified toxin are used and the donor horses produce high levels of antibodies.  The horses are bled when their antibody levels are sufficiently high and the serum antibody fraction is separated from the blood, then processed and dispensed for use in susceptible animals.  Serum antibodies can be introduced into a recipient animal by injection subcutaneously and, or intramuscularly, depending on the product, and in some cases intravenously. Again, the main advantage to passive immunity (serum antibodies) is instant immunity to particular disease(s).  The disadvantage is immunity only lasts 10 –14 days in the recipient, so repeat doses may be needed.  Serum antibodies serve an important role as part of the adjunctive treatment of animals that are currently suffering from a given disease for which the antiserum has antibodies against.  An example would be treating a horse suffering from tetanus with tetanus antitoxin.

Active immunization has several advantages compared to passive immunization.  These include the prolonged period of protection and the recall of boosting of this protective response by repeated injections of vaccine or by exposure to infection.  Ideally, the perfect vaccine should illicit a high protective immune response with the absence of adverse side effects.  This is where the challenge for vaccine manufacturers comes in because these two prerequisites tend to be incompatible in a lot of cases.

Traditionally there have been two types of vaccines through the years; live or modified live vaccines and killed vaccines.  Modified live vaccines infect host cells and undergo replication.  These infected cells then process endogenous (inside the cell) antigen.  In this way the vaccine antigen triggers an immune response.  One of the hazards with this approach is the live vaccine organisms may themselves cause disease or persistent infection (residual virulence).  For this reason the virulence of live vaccines must be reduced so that they will create a protective immune response without creating disease.  The process of reducing virulence is called attenuation.  Attenuation of live vaccines can be achieved in a number of different ways.  One way is adapting organisms to growth in unusual conditions so that they lose their adaption to their usual host.  Genetic manipulation of the organism is another attenuation technique. Growing viruses in cells or species to which they are not naturally adapted.  Prolonged growth in tissue culture in cells that the virus is not adapted to is a common attenuation technique for viral modified live vaccines.  Advantages to using live vaccines include fewer inoculating doses required, adjuvants are unnecessary, less chance of hypersensitivity and, induction of interferon.

Killed or inactivated vaccines are the other common type of vaccine.  If organisms are to be killed for use in vaccine, it is important that they remain as antigenically similar to the living organism as possible. One common way of inactivating is the use of formaldehyde.

Alkylating agents such as ethylene oxide, ethyleneimine, acetylethleneimine, and beta-propiolactone are also used in veterinary medicine as inactivating agents.  The advantages of using killed vaccines are; 1.) They are stable in storage.  2.) They will not cause disease through residual virulence.  3.)  They are unlikely to contain contaminating organisms.  The disadvantages to using killed vaccines include the tendency to induce a lesser immune response then live vaccines and therefore adjuvants are commonly added to killed vaccines to increase the immune response.  Common adjuvants are Aluminum phosphate, Aluminum hydroxide, Freund’s incomplete and complete adjuvant, saponin and many others.  Other disadvantages to killed vaccines are the local reactions that the adjuvants can cause and the need for multiple dosing which increases the risk of hypersensitivity reactions.

The most common route of administering vaccines is through injection subcutaneously or intramuscularly.  Some modified live vaccines can be given intra-nasally to more closely simulate the natural route of infection and stimulate a quicker, local immune response (interferon).  Research using oral vaccines is ongoing and has actually worked with some success in raccoons with an oral rabies vaccine in an edible vehicle.

When a farm or ranch encounters an organism that is unique or a variant that there is no commercial vaccine for, an autogenous vaccine may be useful.  Autogenous vaccines are made from a sample of organisms taken from infected animals on a given farm and is made specifically for use at that particular location.

Although killed and modified live vaccines have been very successful over the years, modern vaccine technology is an exciting and continuously growing field.  Modern genetic techniques can produce new, improved and safe vaccines.  The USDA classifies these new vaccines into 3 categories; Category 1.) Antigens generated by genetic engineering.  Category 2.) Genetically attenuated organisms.  Category 3.) Live recombinant organisms.

The decision to use vaccines for the control of any disease must be based on considerations that the risks of vaccination do not exceed the risk of contracting the disease itself.  In most cases vaccination is the way to go.  As mentioned in the beginning of this article, vaccines have provided a great service to people and animals for decades.  They have helped to eradicate or control many diseases that previously caused epidemics.  Vaccines help the rancher to provide a safe end-product on the grocery store shelves while increasing profitability for the rancher by eliminating diseases.  Vaccines have been so successful in fighting disease through the years that the general public has taken the benefits from vaccinating for granted.  The slight risks of adverse reactions following vaccination should always be viewed in light of the great numbers of animals that have been protected from deadly and costly diseases from vaccination.  Stop and think what the world would be like without vaccines.

Ref.:  Tizard, Veterinary Immunology, 6th edition.