Are we vaccinating too much?

Of course, the answer to this rhetorical question that has been, and is, asked primarily by academicians in various forums during the past several years is supposed to be "yes," otherwise the respondent is considered uninformed, conservative, or worse, simply a greedy capitalist with his or her clients and patients comprising the victimized proletariat. However, the evidence that supports the expected affirmative response, either positive (experimental or epidemiological) or negative (high prevalence of adverse reactions to routine vaccination) continues to be largely wanting.

Setting aside the new canine vaccine guidelines recently published by the American Animal Hospital Association (AAHA), dissection of the textual preamble to the guidelines in this soon-to-be holy writ reveals the (il)logic that is expressly stated or implied for the proposed and largely arbitrary changes. To its credit, the AAHA task force begins with a testimonial to the significance of vaccination in disease prevention. But absent from this soliloquy is the obvious corollary; that, in the case of veterinary medicine at least, the acknowledged success in dramatically reducing the prevalence and incidence of a variety of infectious diseases has been achieved by what started out, admittedly, as an arbitrary protocol, i.e. essentially annual revaccination. The assumption is that divorcing this protocol will have no impact on the success of the procedure (vaccination). What is particularly bothersome is that this assumption has morphed from (implied) hypothesis to scientific truth apparently on the basis of more of the same scantily-clothed expert opinion that, unfortunately, continues to substitute for a scientific approach to the issue of vaccination protocol reformulation.

The AAHA task force indicts "misunderstanding, misinformation and the conservative nature of our profession have largely slowed adoption of protocols advocating decreased frequency of vaccination." It's not clear where the misunderstanding and misinformation come into play, except maybe the misunderstanding and misinformation that its protocol changes are actually based on data. As far as the conservative, slow-to-change nature of the profession goes, it's a good wager that we were probably some of the last to get, and the last to get rid of, polyester leisure suits from our closets. But, if my too-hip preteen daughter's wardrobe is any indication, keeping polyester could be considered visionary. Excluding fashion, there's not necessarily anything wrong with being conservative, especially when contemplating changes to proven procedures. Historically, few scientists have been mainstream conservative thinkers; however, their method, hypothesis testing, versus bending to "expert opinion," is conservative by nature and has stood the test of time since the inception of the scientific revolution in the 1600s. Moreover, any reading of history demonstrates that expert opinion has often been wrong.

Core vs. noncore vaccines

In discussing the definition of core versus noncore vaccines, few would argue that all dogs and cats require vaccination with all available vaccines (antigens) annually. There is certainly much latitude for educated clinical judgment in vaccine administration, which, thankfully for the profession, is why advice on vaccines is best obtained from a veterinarian and not the Internet. But incorporating the possibility of a "response to treatment" in the definition of core versus noncore vaccines would seem a bit more questionable. Many of the diseases that veterinarians and medical doctors 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 health professions is to work to limit clinical disease (morbidity). If this can be done by routine vaccination, i.e. prevention, that would seem to be the choice over treatment, especially at a time when the use of antibiotics in veterinary medicine is coming under greater scrutiny.

Duration of immunity

The AAHA task force states, "Duration of immunity (DOI) is the critical determining factor (in the need for annual vaccination) but it defies simple definition; principally, because it is derived from a complex interplay between the host's immune response and the vaccine in question." What this apparent truism overlooks is the role of microbial population dynamics in affecting DOI in individuals and in pet populations. Increasingly due to urbanization and leash laws, our pets are living in isolation or relative isolation from other members of their species, that is unlike the herds in which humans live. This makes it difficult to extrapolate, as the task force does, from "well-developed epidemiological vigilance in human medicine that indicates immunity induced by vaccination is extremely long lasting and, in most cases, life long." Intuitively, if there is less circulation of infectious agents in a population of widely dispersed animals, then there would be less chance for regular boosting by natural exposure and thereby less effective DOI, compared to individuals living in close contact in a herd. This issue is not addressed in the new recommendations of the task force.

The task force recognizes the concern of many practitioners for "declining patient visits and the missed opportunities to provide health care," but submit "that these guideline provide the canine veterinary practitioner with the opportunity to focus on a more comprehensive wellness program of which vaccines comprise only a small component." Every health professional likely agrees that vaccination is only a part of a wellness program for veterinary and human patients. However, what the guidelines apparently fail to consider, or at least address, is how client compliance will impact, de facto, on the recommended extended vaccination intervals. While there are virtually no data on compliance to vaccination or other procedures in veterinary practice, even if compliance doesn't change as a result of an overall movement to triennial vaccination, the effective DOI is likely to change because there will be more individuals within the population with waning immunity at any given time. Obviously, if overall compliance is effectively reduced by clients more easily "forgetting" about triennial versus annual visits for vaccination, this could have a dramatic effect on herd immunity.

Sprinkled throughout the document are references to the concepts of "(immunological) memory" and "sterile immunity", the latter apparently referring to the ability of a vaccine to prevent infection vs. reduce disease and shedding after infection has occurred. Forgetting for a moment that "sterile immunity" is arguably, at best, a utopian vision of vaccine action, marrying the two concepts results in logical irreconcilable differences. 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. Acknowledging that difference, it would seem conceptually difficult to understand how, in the absence of effector function, that sterile immunity can actually be achieved. Moreover the relative efficacy of effector vs. memory responses in conferring clinical immunity varies with pathogen life style, and also vaccine formulation.

Finally, the endeavor by this task force again raises the question of what we accept as "proof" as a basis for change in the profession. In fairness, this set of guidelines is numerically better referenced than the earlier American Association of Feline Practitioners edict related to extended vaccination intervals that had one reference to a refereed published manuscript. However in this case, there are no referred, published data to support the new recommendations. Certainly the publication process has a lot of warts, notably the politics of the review process; however, it's the best we have as an approach to a standard of accessible scientific proof. While not necessarily disbelieving the conclusions derived from unreferred abstracts, the latter would not seem to constitute a sufficient body of evidence. This goes beyond cantankerous nit-picking, simply because these types of "references" are not transparent to the reader with regard to design, potential biases, sufficiency of experimental subjects and statistical analysis. In fact, most scientific journals do not accept meeting abstracts as citations, for this reason. Why should we?

At the end of the day, there is one apparent incontrovertible "fact" or observation in the vaccine debate; there is an uncanny association between the dramatic reduction in the prevalence and incidence of infectious diseases in companion animals, and "annual" vaccination. Most would argue that this relationship is causal, and further observe that the practice of "annual" vaccination is also associated with only rare acute or chronic adverse reactions in vaccinated populations, en masse. So, perhaps the rhetorical question to be asking in this debate is not, "Are we vaccinating too much?" but rather "If it's not broken, why fix it?"

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.