Risks of Vaccination

While there are certainly risks involved in vaccination, your veterinarian will only recommend vaccines whose benefits far outweigh the risk of vaccine-induced illness or disease. None the less, pet owners should be aware of the risks of vaccination which include, not only immediate adverse reactions, such as nausea or anaphylaxis, but also delayed adverse events including vaccine-associated fibrosarcoma in cats and immune-mediated disease in dogs.

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There are several immediate adverse reactions which occasionally follow vaccination.  Allergic reactions to vaccines are uncommon , but they do occur occasionally, particularly after repeated doses. Signs occur almost immediately after the vaccine is given, and can include skin rashes or hives, difficulty breathing, vomiting, diarrhoea, salivation and collapse.  If the vaccine given is a live weakened or attenuated vaccine there is the possibility that this organism could cause the animal to show mild clinical signs after they have been vaccinated.  Alternatively, the organism could be spread to another animal in which it could cause mild disease.  There is also a possibility that a vaccinated animal could harbour the live organism found in the vaccine over a period of time and later transmit it to a sick or debilitated animal who could become clinically ill.  There is also a risk of the organisms present in live vaccines crossing the placenta and harming the foetuses being carried so live vaccines should be avoided in pregnant animals. Some vaccines cause a local, temporary swelling at the injection site or even a change in the hair colour at the site of a vaccination, this is seen quite often in cats in which the hair at the vaccination site turns white.  Finally, it is possible that a vaccine may not produce adequate immunity in some individuals due to normal variation within the population or immune disorders in certain animals. In addition, there are certain circumstances under which a vaccine may be expected not work properly.  These circumstances include vaccines given to ill animals who’s immune systems are already under a great deal of stress, animals who are currently or have recently undergone treatment with corticosteroids or other immunosuppressive drugs which would prevent the animal’s immune system from responding

As shocking as it may sound, it is a fact that administration of vaccine will induce the formation of tumours in some cats. The cause-and-effect relationship between vaccination and fibrosarcoma in cats has been scientifically established for over a decade and today remains the top vaccination safety concern for veterinarians and cat owners alike. These tumors are known to be aggressive, to have a high rate of recurrence, and metastasize or spread throughout the body. Although the underlying mechanism is not completely understood, there is compelling evidence to support a relationship between post-vaccination inflammation and tumour formation.

The incidence of vaccine-associated sarcoma, which is thought to lie somewhere between 1 in 10,000 and 1 in 1,000 cats vaccinated, is still not known with certainty. Not all cats share equal susceptibility to tumour formation following vaccination and it is thought that genetics contribute to the level of risk. Despite the fact that tumour formation in cats is known to be associated with extrinsic factors such as trauma and the presence of foreign materials such as suture material in addition to vaccination, much of the recent attention given to vaccine-associated fibrosarcoma in cats centers on the role of adjuvant, and adjuvant-induced inflammation.

An adjuvant is a chemical added to killed viral and bacterial vaccines as a means of enhancing the immune response to a relatively weak immunizing antigen such as a killed virus in a vaccine.  Vaccines containing weakened live bacteria or recombinant vaccines containing genetically altered organisms do not contain adjuvants.

A recent study found the occurrence of vaccine-associated sarcoma in cats receiving adjuvanted vaccine to be five times that of cats that only received non-adjuvanted vaccines. It must be noted however, that the administration of only non-adjuvanted vaccines to cats will not completely eliminate the risk of injection-site sarcoma. However, the reduction in tumor risk associated with administering adjuvant-free vaccine to cats would be significant.  

It has also been suggested that a link exists between vaccination and immune-mediated disease in dogs.  Little scientific evidence exists at this time to support this claim, however your veterinarian will be prepared to discuss any concerns that you may have about your pet’s safety.

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