Without question, vaccination has been one of the most important interventions in disease prevention that has ever been developed. In spite of the demonstrable, and ongoing, success of vaccination, a small, but vocal, anti-vaccination movement has developed in human medicine, occasionally buttressed by horrifying instances of adverse reactions, as well as the occasional publications in scientific journals (vis, the Wakefield debacle). Vaccine development continues, and human health continues to benefit, as new vaccines are developed and tested prior to release.

In veterinary medicine, vaccination has also proven to be a boon for animal health. Diseases such as canine parvovirus and canine distemper, feline leukemia, or equine tetanus, have been greatly reduced – in some cases, nearly eliminated – by vaccination. And, as in human medicine, a small, but vocal, anti-vaccination movement has developed, regaling fearful listeners with tales of acute harm, or chronic, low-grade disease (sometimes termed “vaccinosis”).

Yet, in spite of the success of animal vaccination, genuine questions remain as to ideal intervals for booster vaccination. That is, although the effectiveness of vaccination can be shown by reduction in disease, as well as the occasional challenge study (where vaccinated animals are exposed to disease-causing organisms to see if they are protected), no one really knows the “optimum” interval for giving boosters of most animal vaccines.

There are several reasons for this. First and foremost is the fact that animal vaccinations are not subjected to the same rigorous pre-market testing that is required for the release of a human vaccine. Otherwise stated, animal vaccines can be released without large, controlled challenge studies that are necessary prior to the release of human vaccines. Animal vaccines are controlled by the US Department of Agriculture, which merely requires that vaccines be shown to be safe and pure, and have a “reasonable expectation” of efficacy prior to their release; however, the clinical need (relevancy) or usefulness (applicability) of a particular vaccine may not necessarily be assured by the licensing process.

Historically, many animal vaccinations have been given annually. These recommendations were based on the best available knowledge at the time. As research has continued, it appears that some vaccines may protect animals from disease for longer than was previously felt. This information has been incorporated into some vaccine guidelines, e.g., many rabies vaccinations are now given every three years, instead of annually. Still, in spite of good data, new recommendations are not always adopted in a timely fashion, for a variety of reasons, which might include difficulty in disseminating the information, as well as practitioner comfort with schedules that have been effective at disease prevention in the past. Unfortunately, some people have claimed that practitioners resist giving vaccinations less frequently simply due to a profit motive; this assertion is outrageous, and obscures the many difficulties veterinarians face in coming up with an “ideal” program to fit every animal.

As such, it is essentially impossible to make firm, science-based guidelines when it comes to animal vaccination. It is almost without question that “some” vaccination helps prevent animals from disease; it is also almost without question that “no” vaccination puts animals at risk for disease, especially when such animals have contact with other animals; it is fortunate that if animals are vaccinated more frequently than needed to provide protection from disease, there is, in general, little apparent harm. That said, when harm can be shown, it is investigated, e.g., the problem of vaccination-related sarcomas in felines. As a result of such investigations, newer protocols, and newer methods of vaccine administration are being developed.

Unfortunately, there is currently no way to develop an individualized vaccination program for animals that can be said to assure protection from disease and completely prevent adverse reactions. Some people may advocate measuring titers (antibody levels) in the animal’s blood; unfortunately, these levels have not been correlated with disease prevention in animals, that is, having a high titer does not necessarily mean that an animal is protected from disease. Still, one can make some reasonable generalizations, for example, one can be fairly well assured that animals that don’t have contact with other animals may need less vaccination (for example, cats that are kept indoors), or that vaccination of animals at the end of their expected life span may be of less importance. Still, in general, animal owners must currently rely on the experience of individual practitioners, rather than sound science, for vaccine recommendations for their animals.

For more information about animal vaccines, and the vaccine-related controversies, see the Vaccination Principles of the American Veterinary Medical Association, updated in 2007.

Posted by David Ramey

David Ramey, DVM, is a 1983 graduate of Colorado State University.  After completing an internship in equine medicine and surgery at Iowa State University, he entered private equine practice in southern California.  Dr. Ramey is an author of numerous books on equine health care, and a prominent voice for the application of evidence-based standards to veterinary medicine.  He was a member of the task for on "Therapeutic Options" of the American Association of Equine Practitioners, as well as a member of the task force that wrote the current guidelines for the use of "Complementary and Alternative" veterinary medicine for the American Veterinary Association.  He has published numerous articles and books pertaining to "alternative" approaches to veterinary medicine, including the 2004 "Complementary and Alternative Veterinary Medicine Considered," co-authored with world renowned veterinary ethicist Dr. Bernard Rollin.