By Brennen McKenzie, MA, MSc, VMD, cVMA

During the pandemic, the subject of vaccination has been prominent in the popular media. Once the first vaccines for COVID-19 were approved, the subjects of efficacy, safety, herd immunity, and vaccine hesitancy became daily features in the news and the focus of rancorous debates on social media. None of this, of course, is new to veterinarians.

Veterinarians probably have more familiarity with the science and application of vaccines than most physicians, excepting pediatricians. We understand the science of vaccines, the potential risks, and all the issues around immunization that are so hotly debated in the media these days. We also have extensive firsthand experience with the problem of vaccine hesitancy and the misconceptions and misinformation that can discourage appropriate vaccination.^1,2

Debates have raged between veterinarians and pet owners, and within our own profession, about which vaccines to give, when, and how often. There is no immutable, optimal vaccination plan for any specific disease, species, or individual. As scientific evidence is collected, our understanding and practices necessarily evolve. Vaccination looks quite different now than it did when I graduated 20 years ago—and it will undoubtedly look just as different 20 years from now. The current best scientific consensus is expressed in guidance documents from several professional organizations.

The American Animal Hospital Association (AAHA) has produced vaccination guidelines for dogs and, in collaboration with the American Association of Feline Practitioners (AAFP), for cats, as well.^3,4 The World Small Animal Veterinary Association (WSAVA) also has canine and feline vaccination guidelines.⁵ These are good summaries of the available evidence with reasonable generalizations for common practice situations. The guidelines are not intended to be absolute, and they are appropriately modified by institutions and individual veterinarians to account for the characteristics of local populations and specific patients.

However, the necessary flexibility of such guidelines does not suggest all possible alternatives are equally science-based and appropriate. Some organizations and individuals promote dramatically different protocols, which are not rooted in science but incorporate alternative views of health and disease or explicitly anti-vaccine beliefs.

Unfortunately, it is difficult for pet owners to distinguish science-based recommendations from anti-vaccine ideology. As such, veterinarians expend a great deal of time and effort helping owners understand the distinction and make informed choices about vaccination.

Despite such efforts, research suggests perhaps 20 to 50 percent of dogs and cats do not receive appropriate recommended vaccines, and in some areas the vaccination rate is below that required for effective herd immunity against important infectious diseases.⁵

Proponents of unconventional vaccine practices commonly claim vaccines are unnecessary in healthy animals; that a single vaccination provides adequate lifelong protection; that vaccines contain toxins or other dangerous substances; that excessive vaccination overwhelms the immune system and leads to autoimmune disease, cancer, or other health problems; or that vaccine guidelines are driven by the financial rewards for veterinarians more than science and the health of pets. Such claims are the focus of much debate, but they persist even when sound evidence and reasoning shows them to be unfounded.^1,2

Less commonly, proponents of alternative vaccination practices attempt to provide scientific research evidence to support their recommendations. Two prominent examples include a pilot study purporting to show that a "half dose" of distemper and parvovirus vaccine was as effective as the recommended dose in small-breed dogs, and the Rabies Challenge Study, a long-term project attempting to support extending the rabies vaccination interval for dogs.

'Half-dose' vaccines

This study, published in the journal of the American Holistic Veterinary Medical Association (AHVMA), included 13 small-breed dogs recruited through websites and veterinarians affiliated with the association.⁶ Participants were told to give "a ½ dose" of the bivalent canine distemper (CDV)/canine parvovirus (CPV) vaccine. There was no standardization for how this was done, and no measures of antigen content actually delivered to the dogs. Blood samples were taken before vaccination and at four months and six months afterward to measure antibody titers for distemper and parvovirus. No assessment was made of adverse effects or susceptibility to disease.

All dogs had protective titers before being vaccinated. Most had an increase in their titer after vaccination at four months (9/13 for CPV and 11/13 for CDV) and six months (6/8 for CPV and 3/8 for CDV). The author concluded "results of this study confirmed that receiving a half-dose of bivalent DPV vaccine was efficacious."

I have had small-dog owners cite this study as justification for asking me to reduce the volume of vaccine given to their dogs. However, this research is deeply flawed and subject to uncontrolled bias, and it does not in any way support the conclusion that such a volume reduction is safer or equally protective compared with the standard antigen dose shown to be effective through much more rigorous research.

The Rabies Challenge Study

The stated purpose of this study was to demonstrate the duration of protection from rabies vaccination is longer than the three years recommended by the guidelines and to provide support for changing legal rabies vaccination requirements. The known risks of rabies vaccination are uncommon and rarely serious, and there is no evidence that extending the interval beyond three years will reduce these meaningfully without also increasing the risk of rabies.

This study initially included 100 dogs, but only 35 were part of the challenge study directly testing the duration of protection provided by rabies vaccination. Two vaccines were used in the study.

Vaccine A

This was killed, adjuvanted vaccine of the type commonly used for dogs (though the specific vaccine is no longer manufactured). Dogs were vaccinated at 12 weeks of age and again at 15 weeks of age. This differs from the recommended and common practice of vaccinating once at 12 to 16 weeks of age and then again one year later.

At six years and 10 months after the initial vaccination, the vaccinated dogs were exposed to rabies, as were five unvaccinated dogs in the control group. The USDA standard for proving a rabies vaccine good enough for use is that it must protect at least 88 percent of vaccinated dogs. In this trial, all five dogs survived the challenge. However, only 2/5 control dogs developed rabies, showing the virus used for the test was not sufficiently active (natural rabies infection kills 100 percent of infected animals). Therefore, these results can't tell us if the test dogs were truly protected at this time point.

At eight years after final vaccination, another challenge was done with five vaccinated and five unvaccinated dogs. This time, all of the unvaccinated dogs died, showing the test virus was active. However, all but one (4/5 or 80 percent) of the vaccinated dogs also died, so at this time the vaccinated dogs were no longer adequately protected.

Vaccine B

The authors also vaccinated several groups of dogs at the same times with a different rabies vaccines not containing the preservative thimerosal, which is sometimes mistakenly believed to cause health problems.⁷

In the initial challenge study at five years, all vaccinated dogs survived, but so did 60 percent of the unvaccinated dogs. Another five given this vaccine were challenged at six years and seven months, of which 4/5 (80 percent) survived. Another 12 of these dogs were challenged with rabies at seven years and one month, of which 6/12 (50 percent) survived.

Unfortunately, the effort and sacrifice of animals involved in this study did not yield much useful information. The study used two rabies vaccines that are not in common use and an initial vaccination protocol that differs from standard practice, making comparison with conventional practices dubious. The numbers in each challenge group were quite small, and the death or survival of a single animal could drastically change the apparent percentage surviving or dying the challenge, so any generalization about rabies vaccination in general are unsupportable.

The first challenge studies at about five and six years can't be interpreted since most of the unvaccinated dogs survived the virus challenge. Of the dogs challenged at about six-and-a-half years, 20 percent of them died, which is more than the allowable 10 to 12 percent under the USDA guidelines, and certainly not acceptable to veterinarians or pet owners. None of the 10 dogs tested six years after vaccination had a titer above the established protective level. This suggests we would be wise not to wait as long as six years before given rabies boosters to our dogs. Similarly, of the dogs challenged at seven years, 50 percent died, so we certainly should not think of this as an acceptable interval between vaccinations.

Bottom line

Vaccination practices, like all of medicine, inevitably change as circumstances and the scientific evidence evolve. Some disease may be eliminated, and new ones may emerge. Our understanding of the immune system is always improving, and the way we immunize our patients changes accordingly. And vaccine technology advances, making immunization safer and more effective.

There will likely always be those excessively fearful of vaccination, but a rigorous scientific evaluation of the risks and benefits of vaccines continues to demonstrate that the approach recommended in mainstream, science-based guidelines is still the best way to maximize the benefits and minimize the risks of both vaccination and infectious disease for our patients.

Brennen McKenzie, MA, MSc, VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists' opinions do not necessarily reflect those of Veterinary Practice News.

References

1. McKenzie B. Is fear driving the antivaccine movement? Vet Pract News. June 2019:32-33.
2. Mckenzie B. Why Are So Many Pet Owners Not Vaccinating? Adv Small Anim Med Surg. 2020;33(6):1-3. doi:10.1016/J.ASAMS.2020.06.001
3. Stone AES, Brummet GO, Carozza EM, et al. 2020 AAHA/AAFP Feline Vaccination Guidelines. J Am Anim Hosp Assoc. 2020;56(5):249-265. doi:10.5326/JAAHA-MS-7123
4. Ford RB, Larson LJ, McClure KD, Schultz RD, Welborn L V. 2017 AAHA Canine Vaccination Guidelines*. J Am Anim Hosp Assoc. 2017;53(5):243-251. doi:10.5326/JAAHA-MS-6741
5. Day MJ, Horzinek MC, Schultz RD, Squires RA, Vaccination Guidelines Group (VGG) of the World Small Animal Veterinary Association (WSAVA). WSAVA Guidelines for the vaccination of dogs and cats. J Small Anim Pract. 2016;57(1):E1-E45. doi:10.1111/jsap.2_12431
6. Dodds J. Efficacy of a Half-Dose Canine Parvovirus and Distemper Vaccine in Small Adult Dogs: A Pilot Study. J Am Holist Vet Med Assoc. 2015;41(Winter):12-21.
7. Montana M, Verhaeghe P, Ducros C, Terme T, Vanelle P, Rathelot P. Safety review: squalene and thimerosal in vaccines. Therapie. 2010;65(6):533-541. doi:10.2515/therapie/2010069