Vaccination in adult dogs and cats: what do we need to consider?

Vaccines should be thought of as any other medication, and used as necessary to address the true health needs of each canine and feline patient, especially when they become adults. Both the core and non-core revaccination needs of adult dogs and cats should take into account the individual’s risk of exposure, along with geographical location, and lifestyle factors.

A more proactive approach is still needed to both standardize the production and individualize the use of veterinary vaccines to ensure their safety and efficacy. This is especially relevant for dogs and cats who have been properly vaccinated and immunized as youngsters. This article explores factors to consider when creating vaccination strategies for adult animals.

Why a new approach?

Controversies and misunderstandings around the principles of immune memory and protection are ongoing. In addition, compliance failure with current national vaccine policies and guidelines, resistance to change, and denial of the potential for adverse events within the general veterinary community and society as a whole complicate thoughtful vaccine decisions for adult companion animals.

The solution depends upon more focused educational efforts within academic veterinary medicine, clinical practice, and companion animal owner communities. Vaccines are not innocuous products; the benefit/risk equation needs to be assessed before vaccination and booster vaccination.

No panacea

There is little doubt that the application of modern vaccine technology has permitted us to effectively protect companion animals (and people) against serious infectious diseases.1-5 However, vaccinations are increasingly recognized (albeit still rarely) as contributors to immune-mediated blood, skin, bowel, bone, and joint diseases, bone marrow and organ failure, central nervous system excitation, and behavioral aberrations.3,6 Genetic predisposition to these adverse events (termed vaccinosis) has been documented. It must be recognized, however, that we have the luxury of expressing these concerns today only because the risk of disease has been effectively reduced by the widespread use of vaccination programs. Nevertheless, the accumulated evidence indicates that vaccination protocols should no longer be considered a “one size fits all” program.1-3,7,8

In cats, while adverse vaccine reactions may be less commonly seen, aggressive tumors (fibrosarcomas) can occasionally arise at the site of vaccination, as they can in dogs.8,9 Other cancers, such as leukemia, have been also been associated with vaccines.9 In the ensuing 20 years since vaccine-associated sarcomas were recognized in cats, the prevalence has not decreased nor has the age of affected cats increased, despite changes in vaccine formulations and worldwide protocol guidelines.9

Even today, it is estimated that only 40% of veterinarians are following the current World Small Animal Veterinary Association, American Veterinary Medical Association, American Animal Hospital Association, and British Veterinary Association vaccine policy guidelines.3 There is no such thing as an “up to date” or “due” vaccination. Enlightened veterinarians can now offer a package of separated vaccine components, when available, rather than administer them together, since the published data show more adverse reactions when multiple vaccines are administered concurrently.

Vaccine dosage

Dogs are currently vaccinated with the same quantity of vaccine, independent of size or breed.1,2 Why do very small and very large dogs need the same dose of vaccines, when clinical trials by vaccine manufacturers are typically performed on laboratory beagles and with minimal field testing in different breeds prior to licensing?3 More vaccine adverse events have been documented in smaller dogs.2 Toy and small dogs logically should require less vaccine than giant and large dogs in order to be fully immunized. Similarly, puppies and kittens should require less vaccine volume to immunize than adults do.1

In support of the size hypothesis, this author studied healthy, adult, small breed dogs who had not been vaccinated for at least three years.10 The dogs were given a half-dose of bivalent distemper and parvovirus vaccine, whereby all of them developed increased and sustained serum vaccine antibody titers.10 Presumably, this approach would apply also to puppies, and further research is needed.

Vaccinate wisely, only when needed

When an adequate immune memory has already been established, there is little reason to administer booster vaccines, and it would be unwise to introduce unnecessary antigen, adjuvant, and other excipients, as well as preservatives, by doing so.3-5 Serum antibody titers can be measured triennially or more often if needed, to assess whether a given animal’s humoral immune response has fallen below levels of adequate immune memory. In that event, an appropriate vaccine booster can be administered. For legally required rabies vaccines, these alternative options are often limited by necessity, given the exposure risk to this fatal disease.11

Importantly, pet caregivers should understand that the act of giving a vaccine may not equate to immunization. Vaccines may not always produce the needed or desired immune protective response. This can occur if the vaccine itself was inadequately prepared (very rare) but also if the pet is a genetic low- or non-responder to that vaccine (quite common in certain breeds of dogs and their families, such as Akitas, Greyhounds, and Labrador Retrievers, especially black Labs). In the latter case, that pet will be susceptible lifelong to the disease of concern and revaccination will not help and could even be harmful.3,7

Vaccination can provide an immune response that is similar in duration to that which follows a natural infection. In general, adaptive immunity to viruses develops earliest and is highly effective. Such antiviral immune responses often result in the development of sterile immunity and the duration of immunity (DOI) is often lifelong.3,7,10 In contrast, adaptive immunity to bacteria, fungi, or parasites develops more slowly. The DOI is generally short compared with most systemic viral infections. Sterile immunity to these infectious agents is less commonly engendered.

Old dogs and cats rarely die from vaccine-preventable infectious disease, especially when they have been vaccinated and immunized as young adults (i.e. between 16 weeks and one year of age).7,10 However, young animals do die, often because vaccines were either not given or not given at an appropriate age (e.g. too early in life in the presence of maternally-derived antibody).1-3

More animals need to be vaccinated to achieve the 70% or more needed to increase herd (population) immunity to protect the unvaccinated against these diseases.2,8 Titers do not distinguish between immunity generated by vaccination and/or exposure to the disease, although the magnitude of immunity produced just by vaccination is usually lower.

Core vaccines should be given to all adults, but not more often than every three years, and we prefer to measure serum antibody titers instead.1-3,7 Serological and challenge studies actually indicate that protection is likely much longer, ranging from seven to nine years.7

Other issues with over-vaccination

Increased costs in time and dollars need to be considered, despite the well-intentioned solicitation of clients for pets to receive their wellness examinations.1,3 Giving unnecessary annual boosters has the client paying for services that are likely of little benefit to the pet’s existing level of protection against these infectious diseases. It also increases the risk of adverse reactions from the repeated exposure to foreign substances.1-4

Compliance or resistance to current vaccine guidelines

The issues discussed above have been legitimately raised for over two decades, but why is this knowledge still considered controversial?1-3,8 Have veterinarians embraced the national and international policies on vaccination guidelines? Does the public trust veterinarians to be up-to-date on these issues? Do they believe veterinarians have a conflict of interest if they derive income from annual booster vaccinations? While some veterinarians still tell their clients there is no scientific evidence linking vaccinations with adverse effects and serious illness, this fallacy confuses an impressionable client.3 On the other hand, vaccine and anti-vaccine zealots abound with hysteria and misinformation. Neither of these polarized views is helpful.

Veterinary practitioners may simply believe what they originally learned about vaccines and are therefore less inclined to change or “fix” what is perceived to be unbroken.1-3,8 Annual vaccination has been the single most important reason why the majority of people bring their dogs and cats for an annual check-up or “wellness visit”.  When combined with a failure to understand the principles of vaccinal immunity, it is not surprising that attempts to change vaccines and vaccination programs have created significant controversy.

As stated by the American Animal Hospital Association’s 2003 guidelines: “No vaccine is always safe, no vaccine is always protective, and no vaccine is always indicated. Misunderstanding, misinformation, and the conservative nature of our profession have largely slowed adoption of protocols advocating decreased frequency of vaccination. Immunological memory provides durations of immunity for core infectious diseases that far exceed the traditional recommendations for annual vaccination. This is supported by a growing body of veterinary information as well as well-developed epidemiological vigilance in human medicine that indicates immunity induced by vaccination is extremely long lasting and, in most cases, lifelong.”1-3 These statements were groundbreaking at the time, and still apply today.3

Vaccines should be individualized to each patient

“Vaccination should be just one part of a holistic preventive healthcare program for pets that is most simply delivered within the framework of an annual health check consultation,” said the late Professor Michael J. Day. “Vaccination is an act of veterinary science that should be considered as individualized medicine, tailored for the needs of the individual pet, and delivered as one part of a preventive medicine program in an annual health check visit.”1 


1Day M J, Horzinek MC, Schultz R D, Squires R. “WSAVA Guidelines for the vaccination of dogs and cats”. J  Sm Anim  Pract  2016; 57: E1-E45.

2American Animal Hospital Association (AAHA). Canine Vaccination Task Force: Ford RB, Larson LJ, Schultz RD, Welborn LV. “2017 AAHA canine vaccination guidelines”. J Am Anim Hosp Assoc 2017; October: 26-35.

3Dodds WJ. “Vaccine issues and the World Small Animal Veterinary Association (WSAVA) Guidelines (2015-2017).”  Israel J Vet Med 2018; 73 (2): 3-10.

4Dodds WJ, Herman K. “Heavy Metals in vaccines”. J Am Hol Vet Med Assoc 2019; 57: Winter 16-18.

5Dodds WJ. “Adjuvants and additives in human and animal vaccines”. Med Res Arch. 2016; 2(5): 1-8.

6Dodds WJ. “Rabies virus protection issues and therapy”. Global Vaccines Immunol. 2016; 1: 51-54.

7Schultz RDThiel BMukhtar ESharp PLarson LJ. “Age and long-term protective immunity in dogs and cats”. J Comp Pathol. 2010; Jan;142 Suppl 1: S102-8.

8Scherk MA (Chair), et al. 2013 AAFP Feline Vaccination Advisory Panel Report. J Fel Med Surg 2013; 15:785-808.

9Wilcock B, Wilcock A, Bottoms K. Brief communication. Feline postvaccinal sarcoma: 20 years later. Can Vet J 2012; 53: 430-434.

10Dodds WJ.  Efficacy of a half-dose canine parvovirus and distemper vaccine in small adult dogs: a pilot study.    J Am Hol Vet Med Assoc  2015; 41:12-21.

11Dodds WJ, Larson LJ, Christin, KL, Schultz RD. Duration of immunity after rabies vaccination in dogs: The Rabies Challenge Fund research study. Can J Vet Res 2020; 84:153–158.


Dr. Jean Dodds received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she established Hemopet, the first non-profit national blood bank program for animals. Today, Hemopet also runs Hemolife, an international veterinary specialty diagnostics service. Dr. Dodds has been a member of many committees on hematology, animal models of human disease and veterinary medicine. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1994, has served two terms on the AHVMA’s Board of Directors, chairs their Communications Committee, and currently serves on the Board of the AHVMF, as well as its Research Grant and Editorial Committees.


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