Vaccines are one of the most effective means for preventing disease ever developed and one of the great triumphs of scientific medicine. Widespread vaccination of children has dramatically reduced—and in some cases eliminated—infectious diseases that have plagued humanity for thousands of years. There is less research evidence for the impact of vaccination in companion animals, but there is ample reason to believe vaccines have been equally successful in dogs and cats.3–7 Any veterinarian old enough to remember the emergence of canine parvovirus (CPV) in the U.S. in the late 1970s, for example, would probably testify to the efficacy of vaccination in reducing the incidence of this disease.5–7 There also is compelling evidence showing the reduction of both canine and human rabies cases due to vaccination programs aimed at dogs.8–12 And similar to smallpox, the veterinary disease rinderpest was eradicated largely due to the use of an effective vaccine.13 Vaccines and autism Despite the evident success of vaccination in reducing morbidity and mortality, there has always been controversy about the practice.14 The introduction of widespread vaccination in children was initially met with resistance and legal challenges. There was fear about the safety of vaccines, as well as resistance rooted in political and religious beliefs. This resistance was diminished by the dramatic reduction in childhood disease accomplished by vaccination programs in the early 20th century. With the elimination of smallpox, the near elimination of polio, and dramatic reductions in many common and familiar childhood diseases, the vast majority of parents embraced routine vaccination. Today, however, most parents belong to generations that have grown up in a world largely free of vaccine-preventable infections. They are unfamiliar with diseases their grandparents knew all too well, so their concern about them is low, while fear of vaccines and resistance to vaccination is growing again.14–16 The latest surge in the modern anti-vaccine movement can be linked to a scientific paper suggesting the measles, mumps, and rubella vaccine (MMR) increased the risk of autism in children.17 This paper has since been withdrawn due to fraud, the author has been stripped of his medical license, and numerous studies have definitively disproven any link between autism and vaccination.18–23 Nevertheless, the avalanche of fear set off by such a claim continues, and outbreaks of vaccine-preventable diseases are occurring in children around the world associated with declining vaccination rates.16,24,25 The anti-vaccine movement has not been limited to human medicine. There are all too many activists discouraging vaccination of dogs and cats, and blaming vaccines for a panoply of diseases.26–29 Fads and irrational ideas about health pertaining to humans often get adopted and translated to pet health care. Phobia about gluten and grains in human diets undoubtedly contributed to the grain-free pet food craze.30 And fears of the ill-defined menace of “chemicals,” genetically modified organism (GMO) foods, and other supposed health hazards for people also infect pet owners and veterinary professionals.30–32 As an extreme example, some anti-vaccine activists have made the claim that vaccines cause autism in dogs.33,34 While the concept of an animal model for autism as a neurologic disease is not inherently unreasonable, there is no definitive evidence a condition analogous to autism exists in dogs.35,36 Of course, even if it does, there is absolutely no reason to associate any such condition with vaccination. This is clearly an anti-vaccine argument originally developed for children and then adopted as an argument against vaccination of pets as well. What are the risks? Like any medical intervention, vaccines in pets are not entirely free of risks.6,37–43 Adverse effects ranging from acute hypersensitivity reactions to vaccination and to serious and delayed diseases, such as feline injection-site sarcoma, do occur. Most of these are either mild and treatable or very uncommon, but vaccines clearly have some dangers. For most of the diseases blamed on vaccines, however, the evidence ranges from weak (e.g. immune-mediated thrombocytopenia and hemolytic anemia)37–41 to nonexistent (e.g. vaccinosis, a catch-all concept invented by a homeopath in the 19th century and still used to blame chronic diseases of all sorts on vaccination).44–46 The risks of vaccinations must be assessed in comparison to their benefits, and it is obvious core canine and feline vaccines, used appropriately, offer more than enough benefits to justify their risks. Sadly, anti-vaccine propaganda comes not only from laypeople, but also from some within the veterinary profession.46–52 Reasonable, evidence-based inquiry into the risks of vaccination and the proper use of vaccines is appropriate and necessary. However, vaccine opponents often engage in fear-mongering and exaggeration of the risks (see the sidebar in the XXXX of this page), or in the promotion of unproven or ineffective alternative immunization practices. Many of the most vociferous opponents of vaccination also are proponents of alternative medicine. Research has consistently found an association between vaccine refusal and the use of alternative therapies in humans.53–60 Parents who use these methods are less likely to follow recommended vaccination schedules. These parents often explain their resistance to vaccines as part of a comprehensive ideological preference for supposedly “natural” health care and a suspicion of technology and modern scientific medicine. Alternative medicine providers, such as naturopaths, chiropractors, and homeopaths, also frequently discourage vaccination.53,55–59,61–66 Though there is a lack of formal research on this subject in veterinary medicine, a similar relationship is often apparent in the pronouncements of vaccine opponents. The strongest resistance to routine vaccination comes from advocates for alternatives to conventional science-based medicine, and these activists also frequently oppose other elements of conventional treatment. Certainly, there are integrative practitioners who support vaccination, but the resistance to vaccines also comes predominantly from pet owners and veterinarians sympathetic to alternative medicine. In human health, the harmful effects of the anti-vaccine movement are clear. Outbreaks of measles, whooping cough, and other preventable childhood diseases are occurring with increasing frequency, and these are driven predominantly by unvaccinated individuals.24,25,67 Avoidable suffering and death have been caused by the use of excessive and irrational fear of vaccines to discourage parents from properly immunizing their children. There is little data to determine the impact of the anti-vaccine movement on vaccination rates or infectious disease incidence in dogs and cats. Vaccine uptake is well below levels needed for effective herd immunity in some populations, but the cause for this is unclear.68–71 Media reports also suggest vaccine resistance is a growing phenomenon among certain groups of pet owners, but there is no clear research data evaluating this claim.26,28,29 A few surveys of pet owners regarding vaccines have provided variable results, with some finding concerns about vaccine safety discourages vaccination and others not identifying this as a significant factor.69–74 It seems likely the causes and effects of vaccine resistance in veterinary medicine resemble those in human health care, but there is no strong evidence to demonstrate this. Where human and pet health collide Veterinary medicine often borrows from human medicine. The best version of this practice is when new knowledge and novel therapies discovered for people are studied and ultimately adapted to improve the health of veterinary patients. Unfortunately, pet owners and veterinary professionals also sometimes adopt and apply bad ideas from the human health field that can undermine effective, evidence-based veterinary care. The excessive fear of vaccines and the decline in vaccination of companion animals appear to be examples of irrational and unscientific health-care practices bleeding over from the human to the veterinary health-care domain. It is incumbent on veterinarians to be aware of the fears and misconceptions pet owners, and even some of our colleagues, may have concerning vaccines and to promote more evidence-based views and practices. Vaccines have protected the lives and health of countless veterinary patients, and we should resist efforts to undermine the use of this powerful health-care tool. 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 Vanderslott S, Roser M. Vaccination. OurWorldInData. 2 Roush SW, Murphy T V, Vaccine-Preventable Disease Table Working Group. Historical Comparisons of Morbidity and Mortality for Vaccine-Preventable Diseases in the United States. JAMA. 2007;298(18):2155. doi:10.1001/jama.298.18.2155 3 Patel JR, Heldens JGM. Review of companion animal viral diseases and immunoprophylaxis. Vaccine. 2009;27(4):491-504. doi:10.1016/j.vaccine.2008.11.027 4 McVey S, Shi J. Vaccines in Veterinary Medicine: A Brief Review of History and Technology. Vet Clin North Am Small Anim Pract. 2010;40(3):381-392. doi:10.1016/j.cvsm.2010.02.001 5 Houston DM, Ribble CS, Head LL. Risk factors associated with parvovirus enteritis in dogs: 283 cases (1982-1991). J Am Vet Med Assoc. 1996;208(4):542-546. http://www.ncbi.nlm.nih.gov/pubmed/8603904. Accessed March 17, 2019. 6 Appel MJ. Forty years of canine vaccination. Adv Vet Med. 1999;41:309-324. http://www.ncbi.nlm.nih.gov/pubmed/9890024. Accessed March 17, 2019. 7 Brady S, Norris JM, Kelman M, Ward MP. Canine parvovirus in Australia: The role of socio-economic factors in disease clusters. Vet J. 2012;193(2):522-528. doi:10.1016/j.tvjl.2012.01.025 8 Zinsstag J, Lechenne M, Laager M, et al. Vaccination of dogs in an African city interrupts rabies transmission and reduces human exposure. 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