Principles of Vaccination

Editor's Note: This is Chapter 1 of Understanding Equine Preventive Medicine by author and veterinarian Bradford G. Bentz, VMD. The book is available from

Vaccination plays a major part of a preventive medicine program, but many programs for individual horses and even for large stables are often based on incomplete understanding of the principles of vaccination and the particular needs of the animals for which the program is designed. For these reasons it is important to have realistic expectation based on a sound understanding of what particular vaccination programs provide.

Vaccination minimizes the risk of infection but does not prevent disease in all circumstances. To be effective, a primary series of vaccinations with a complete booster series must be administered prior to exposure to an infectious disease.

It should be noted, though, that not all horses respond similarly to a vaccine, and the duration of immunity against a particular infectious agent can vary.  A herd approach to vaccination rather than an individual approach should be taken to control disease. This is extremely important in any barn in which diseases can be directly transmitted among horses.  The time required to produce effective immunity from vaccination using a federally licensed vaccine is usually about two to three weeks after the primary series is completed, and one or more weeks after the routine booster is administered.

Presently, the efficacy of many of the commercially available equine vaccines is questionable. Indeed, few controlled studies have been conducted to evaluate their efficacy. Therefore, the use of vaccines should be considered only one component of the overall preventive medicine program aimed at limiting the incidence of disease.  The other important components involve the use of all available information pertaining to various disease risk factors, modes of transmission, methods of early detection, appropriate quarantine of animals affected with contagious diseases, and good management practices aimed at preventing the transmission of disease by people or implements involved in the care of the affected horses.  These have been outlined in the previous chapter.

The use of vaccines that are directed against more than one antigen, or multivalent vaccines, are common in many, if not most, equine practices.  Despite their convenience, some very valid concerns should be considered.  Use of multivalent vaccines presumes that there is one best time to vaccinate against all of the antigens contained within the vaccine. However, the periods of exposure to the disease-causing pathogens against which the vaccine is meant to protect may differ.  Thus, the immunity produced and the height of protection may not occur when the risk of disease is greatest.

Secondly, the administration of multivalent vaccines also presumes that the same vaccine manufacturer produces the most effective vaccines against all of the diseases contained therein.  Another major problem is that the duration of immunity for many of the vaccines contained within the multivalent vaccine is likely to be quite different.  Therefore, administration of one multivalent vaccine twice a year may be adequate for coverage against one of the diseases, while immunity may have dissipated many months ago for another.

Finally, different horses are at different risk for various diseases.  A "five-way" multivalent vaccine might contain one or more vaccines against a disease unlikely to be seen in an isolated horse or even on a small isolated farm.  Meanwhile, the performance horse that is shipping frequently for competitions and encountering many other horses with unknown vaccination status from other areas is more likely to encounter less common diseases.

About the Author

Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine)

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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