A Perspective on Changes in Vaccine Protocols

Annual vaccination and wellness exams have dramatically reduced the levels of infectious diseases in dogs and cats in the United States. Since annual vaccination is considered to be the gold standard, many veterinarians are puzzled and confused by the recent suggested changes for vaccinating dogs and cats. The recent publication of the canine vaccine guidelines by the American Animal Hospital Association (AAHA) proposes changing how veterinarians vaccinate dogs, but does not offer any new information or research on the need or justification for these revisions.

Veterinary biologics firms, universities and veterinarians share some of the same objectives. Certainly, none of these are philanthropic organizations. They're all businesses, at some level, so, of course, there is an economic incentive that comes into play with this discussion on vaccines. However, after removing any commercial or economic concerns from this debate, there still remains an array of issues to address.

The conceptual framework of the guidelines, which serves as the foundation of this document and impetus for change, is flawed. A sturdy framework is essential to the strength and integrity of a structure, and this holds true for the new AAHA canine vaccine guidelines as well.

The need for vaccine protocol change has served as a topic of debate among veterinary experts since the mid 1990s. The first discussions began with anecdotal observations and opinions from experts. These apparently have morphed into largely undocumented recommendations that are the stuff of reality on Web sites on the Internet. The veterinary profession should not accept this as a reasonable process for change. From a scientific perspective, data should confirm or contradict a hypothesis; when the data do not fit the hypothesis, then the hypothesis should be rejected, not the data. The same can be said for vaccine guidelines released for both cats and dogs. The guidelines released by the American Association of Feline Practitioners (AAFP) and AAHA both have questionable supporting data. Excluding the rehashed abstracts from proceedings and opinion papers, there remains no substantial evidence that supports change in terms of (increased) prevalence of adverse reactions or new information on duration of immunity (DOI).

Veterinarians need to use their clinical judgment when determining the vaccination protocol for each individual pet, says author Dr. John Ellis.

The AAHA task force that developed the canine vaccine guidelines alluded to the historical justification for vaccines use, but omitted any reference to the (successful) frequency of vaccination, i.e. the recommendation for annual vaccination. From a medical standpoint, it cannot be assumed that changing the frequency of use of a procedure will necessarily produce the same results. However, the AAHA task force is asking veterinarians to make that conclusion in its recommendations on vaccination.

Veterinarians need to use their clinical judgment when determining the vaccination protocol for each individual pet. For example, we can use the data Dr. Philip Kass presented on his study regarding the risk factors associated with the development of vaccine-associated sarcomas in cats to make a clinical judgment (See related story, p. 7). Let's assume, for the sake of argument, that the data Kass presented in his study were in error by a factor of 10 for the prevalence of adverse reactions following vaccination. That would imply that 1 percent of the population of vaccinated animals have some sort of adverse reaction. That would be a high prevalence. Nevertheless, in that unlikely event, the rational clinical judgment would be not to vaccinate that 1 animal in 100 again with the same vaccine. The irrational approach would be to extrapolate that reticence to vaccinate to the other 99 percent of the population that doesn't have the adverse reaction, and make recommendations to fit the population on that basis.

Clinical judgment is also a factor when discussing core versus noncore vaccines. Few veterinarians or veterinary experts would argue that all dogs and cats require vaccination with all available vaccines annually. There is certainly much leeway for educated clinical judgment in vaccine administration. But considering the possibility of a response to treatment as a definition for core versus noncore vaccines is questionable. Many of the diseases that veterinarians vaccinate for do not cause severe disease or death in the majority of susceptible animals in a population, raising the question, is it worthwhile to use vaccination to reduce morbidity? Assumedly, one of the goals of veterinary medicine is to work to limit clinical disease, i.e. reduce morbidity. Routine vaccination, i.e. prevention, would seem to be the preferable course of action rather than treatment, especially at a time when the use of antibiotics in veterinary medicine is coming under greater scrutiny. It is therefore not rational, or responsible, to decide if an animal should be vaccinated or not depending on whether the disease is treatable or not. This shouldn't be the litmus test for vaccinating. For example, there is no treatment for the West Nile virus infection, so the only course of action is to prevent the disease, by vaccinating horses. However, in the case of Bordetellosis, a treatable disease, should the course of action be to wait until a dog (or cat) contracts the infection in a high-risk environment and becomes clinically ill, and then attempt to treat the disease, or, rather, to attempt to prevent it through routine vaccination?

Attempts have been made by the AAHA task force to relate vaccine administration in humans to animals. Research in human medicine indicates that people don't need to be vaccinated every year, so why do we need to vaccinate dogs every year (to achieve optimal clinical responses)? Maybe because dogs live in a very different environment compared to humans. Certainly, the duration of immunity (DOI) of a vaccine involves the complex interplay between the animal's (host) response and the vaccine. But, the role of microbial population dynamics in pet populations also impacts the effective DOI. However, this is not mentioned in the AAHA guidelines. Humans spend a great deal of their time interacting with one another, often, in closely confined spaces. When people have regular contact with one another, their immune systems experience a natural "boosting" that results from exposure to a variety of endemic infectious agents.

Increasingly, this isn't true for most pets. With urbanization and leash laws, many dogs and cats now have little, if any, interaction with other members of their species. They are living in relative isolation which is very different from the way that they evolved. When animals become widely dispersed and have limited interaction, it's only logical there is less of a chance for regular boosting by natural exposure, and, resultantly, their immune systems may respond quite differently to infectious agents. As a result, the DOI of a vaccine may be shortened, compared to that of a similar vaccine used in people living in close proximity, essentially, as in a herd.

Sprinkled throughout the AAHA guidelines are references to "immune memory" and "sterile immunity" and how these concepts operate in tandem. The guidelines refer to "sterile immunity" as a vaccine's ability to prevent infection and immune memory as the mechanism that affects this protection. However, from an immunological standpoint, these two concepts are often mutually exclusive. Immune memory is a state in which there are minimal or undetectable effector cells, those cells making antibody or mediating antigen specific cell-mediated immune responses, and a preponderance of "memory" cells, those "immature" lymphocytes that are phenotypically and functionally different from effector cells and expand clonally to develop into effector cells upon re-exposure to antigen. Recognizing that difference, it would seem conceptually difficult to understand how, in the absence of effector function, that sterile immunity can be achieved. Moreover, the relative efficacy of effector versus memory responses in conferring clinical immunity will vary with pathogen life style, and also vaccine formulation.

Although the AAHA task force recognizes veterinarians are concerned about declining patient visits and the chance to provide preventive health care for pets, neither the AAHA nor AAFP guidelines address the issue of compliance. Practitioners will agree vaccinations are only one component of veterinary wellness care, but will changing vaccine protocols impact compliance? There are very few data on vaccine compliance, or other procedures in veterinary medicine. Changing the recommended frequency of vaccine administration may have indirect impact on the DOI of vaccines because there may be more animals in a population with waning immunity at any one time. Moreover, compliance could be effectively reduced by clients "forgetting" about or missing triennial visits for vaccination when they were accustomed to annual vaccination programs. These "sociological" factors could have a dramatic effect on herd immunity in pet populations.

After considering these points of discussion, practitioners as analytic thinkers and scientists need to ask themselves, "Is there enough proof to support the recommendations for change?" Has the correct null hypothesis been tested? In other words, has it been demonstrated that the same (high) level of clinical immunity that has been achieved through the recommendation for annual vaccination result if there is a change, en masse, to triennial or longer vaccination intervals? We simply do not know. Annual vaccinations have prevented massive outbreaks of infectious disease for years, and this has been achieved with rare or uncommon adverse reactions in pet populations as a whole. Why then are we trying to fix something that isn't broken?

Dr. Ellis received his veterinary degree from the University of Illinois in 1979. He also completed his Ph.D. in comparative pathology in 1984 from Colorado State University He is a diplomate of the American College of Veterinary Pathologists and the American College of Veterinary Microbiologists in both immunology and virology.

Dr. Ellis is a professor in the department of veterinary microbiology at The University of Saskatchewan. He has written numerous peer reviewed articles in the areas of virology and immunology and is a noted lecturer on these subjects.