Types of Vaccines
Vaccination remains a cornerstone of disease prevention in veterinary medicine. However, increasing concern around the frequency and type of vaccines administered has led to a reassessment of standard protocols, particularly in relation to over-vaccination and adverse effects. A scientific understanding of vaccine types and their immunological impacts is essential for informed decision-making.
What Is a Vaccine?
A vaccine is a biological preparation designed to stimulate the immune system to recognise and respond to specific pathogens, thereby reducing the risk or severity of disease. The goal is to induce immunological memory, allowing the body to mount a rapid and effective defence upon future exposure to the same pathogen.
Vaccines used in veterinary medicine fall into three main categories:
1. Inactivated (Killed) Vaccines
Inactivated vaccines contain virus particles, bacteria, or other pathogens that have been grown in culture and then rendered non-infectious through chemical or physical methods. These vaccines cannot replicate or cause disease in the recipient.
Because inactivated pathogens generally elicit a weaker immune response than live ones, adjuvants are commonly added. Adjuvants—such as aluminium salts or oil emulsions—enhance antigen presentation and increase immunogenicity. However, their inclusion is associated with a higher risk of vaccine-associated adverse effects, including inflammation, hypersensitivity, and, in rare cases, autoimmune disease.
2. Modified Live (Attenuated) Vaccines
Modified live vaccines are derived from naturally occurring pathogens that have been attenuated, or weakened, in the laboratory. These vaccines retain the ability to replicate in the host but have lost the capacity to cause clinical disease under normal conditions.
Because they more closely mimic natural infection, modified live vaccines generally produce a strong, durable immune response. They usually do not require adjuvants and are associated with fewer inflammatory side effects. However, they are contraindicated in immunocompromised animals and may cause mild clinical signs.
3. Recombinant Vaccines
Recombinant vaccines are produced using genetic engineering techniques. Specific genes encoding antigenic proteins of a pathogen are inserted into a non-pathogenic carrier organism—commonly yeast or bacteria—which is then cultured to produce the target protein in large quantities. The purified protein is injected into the animal to stimulate an immune response.
Recombinant vaccines offer advantages in safety and precision. They do not contain whole organisms, are non-replicating, and can be designed to exclude unnecessary or reactive components. In some cases, they can also be adjuvant-free. This class of vaccine is considered particularly useful for animals with prior adverse vaccine reactions.
Concerns Regarding Over-Vaccination
While vaccination is essential to disease control, repeated administration of vaccines that are not clinically necessary carries risks. Over-vaccination may:
Trigger hypersensitivity reactions
Promote autoimmune disease
Induce chronic inflammation
Overwhelm or dysregulate the developing immune system in young animals
Studies suggest that some vaccines, especially those targeting viral diseases, confer long-term or even lifelong immunity after the initial series. Despite this, annual re-vaccination continues to be a common practice in some regions. This approach does not account for individual immune status or breed-specific sensitivities and may result in unnecessary exposure to vaccine components, especially adjuvants.
Titre testing (measurement of circulating antibodies) offers a more targeted approach by assessing the presence of protective immunity. This can help determine whether re-vaccination is necessary on a case-by-case basis.
Adjuvants and Risk of Adverse Events
Adjuvants play a critical role in inactivated vaccines but also increase the potential for local and systemic reactions. Aluminium and mercury-based compounds are the most common adjuvants in veterinary vaccines. Their use has been linked to:
Injection-site inflammation
Granulomas
Immune-mediated disease
Vaccine-associated sarcoma in cats (fibrosarcoma at injection sites)
In cats, the shift in injection site from the intrascapular region to distal limbs was implemented not to reduce the incidence of sarcoma, but to allow easier surgical management (i.e. limb amputation) in the event of tumour development.
Conclusion
Veterinary vaccines are critical tools in disease prevention but are not without risk. A deeper understanding of the types of vaccines used, their mechanisms of action, and their associated risk profiles allows for more informed clinical decisions. When possible, selecting vaccines based on individual need, using titre testing to guide re-vaccination, and avoiding unnecessary adjuvants can reduce the potential for harm.
Veterinary practitioners should evaluate lifestyle, regional disease prevalence, and patient history when developing vaccination protocols. Guardians are encouraged to discuss available options—including recombinant or non-adjuvanted vaccines—when appropriate.
Reference
Moore GE, Guptill LF, Ward MP, et al. Adverse events diagnosed within three days of vaccine administration in dogs. J Am Vet Med Assoc. 2005;227(7):1102-1108.
🔗 https://doi.org/10.2460/javma.2005.227.1102