Evaluating Vaccine Risks
Vaccination plays an essential role in the prevention of infectious diseases in dogs and cats. Core vaccines—those protecting against highly contagious and life-threatening diseases—are widely regarded as necessary for animal health. However, growing research suggests that the repeated administration of vaccines beyond what is needed for protective immunity may contribute to adverse effects, particularly when adjuvants are used.
Immunity and Re-Vaccination
Not all vaccines induce immunity of equal duration. Some vaccines, particularly those against viral pathogens such as canine distemper, parvovirus, and adenovirus, often confer long-term or even lifelong immunity after an initial series. In contrast, other vaccines—such as those for bacterial infections like leptospirosis—may produce a shorter duration of immunity and require more frequent administration.
Role of Adjuvants in Vaccine Reactivity
To enhance the immune response and increase the duration of protection, some vaccines—especially inactivated (killed) vaccines—contain adjuvants. Common adjuvants include:
Aluminium salts
Thimerosal (a mercury-based preservative)
While adjuvants improve vaccine efficacy, they also contribute to increased incidence of vaccine-associated adverse events, including:
Local inflammation
Hypersensitivity reactions
Autoimmune diseases
Granulomas or sterile abscesses
Notably, inactivated vaccines with adjuvants account for approximately 85% of documented vaccine reactions in companion animals. The rabies vaccine, which in many jurisdictions is legally required on a routine basis, is one of the most frequently reported sources of such reactions.
Rabies Vaccine and Regional Mandates
In the United States and Canada, rabies vaccination is legally mandated in most regions. Adverse reactions following rabies vaccination include:
Vomiting, diarrhoea, or fever
Behavioural changes
Seizures
Anaphylaxis
Chronic inflammation or neurological dysfunction
Some reactions may be linked specifically to the aluminium and mercury compounds in adjuvanted rabies vaccines. While non-adjuvanted rabies vaccines are available for cats in some countries, including the United States, they are not widely available for dogs.
Vaccine-Associated Sarcoma in Cats
Cats are especially sensitive to vaccine-related adverse effects. One of the most serious concerns is the development of vaccine-associated sarcoma (VAS), a malignant tumour at the injection site. Although relatively rare, these tumours are aggressive and invasive.
To facilitate surgical intervention if needed, veterinary practice has shifted injection sites from the intrascapular region (back of the neck) to the distal limbs or tail. While this strategy does not reduce the risk of sarcoma formation, it allows for amputation with clearer surgical margins if necessary.
Immunological Risks of Over-Vaccination
Over-vaccination has been associated with several immune-mediated conditions in dogs and cats, including:
Immune-mediated haemolytic anaemia
Immune-mediated thrombocytopaenia
Polyarthritis
Cutaneous and systemic vasculitis
Thyroiditis
Repeated stimulation of the immune system through unnecessary vaccination may also contribute to chronic inflammatory states or hypersensitivity responses. The impact of administering vaccines to very young animals whose immune systems are not fully mature is also under investigation, with concern about long-term immune dysregulation.
Considerations for a Balanced Approach
To minimise risk while maintaining protection against serious disease, a more individualised approach is warranted. Key considerations include:
Assessing risk based on environment and lifestyle
Using antibody titre testing to evaluate immunity
Choosing non-adjuvanted or thimerosal-free vaccines where available
Avoiding unnecessary vaccination in animals with prior adverse reactions
Spacing vaccinations appropriately rather than administering multiple vaccines simultaneously
Discussing these options with a veterinarian who is experienced in risk-based vaccination strategies can help ensure that pets receive appropriate protection with minimal risk.
Vaccination remains a critical tool in disease prevention, but growing evidence supports the need to reevaluate standard vaccination protocols, particularly regarding frequency and formulation. While core vaccines are essential, repeated and unnecessary administration—especially of adjuvanted vaccines—may contribute to a range of adverse outcomes. Veterinarians and pet guardians can work together to develop a tailored vaccination plan based on scientific evidence, lifestyle risk, and regional requirements.
Reference
Moore, G. E., et al. (2005, USA). Adverse events diagnosed within three days of vaccine administration in dogs. Journal of the American Veterinary Medical Association, 227(7).
🔗 https://doi.org/10.2460/javma.2005.227.1102