We know that whether to vaccinate your cat or not is about the balance between benefits and risks. It is interesting to know what the risks are.
The first point of note, based upon a report in the book The Welfare of Cats (section written by K. Sturgess), is that vaccine reactions are the most common adverse drug reaction reported to the Veterinary Medicines Directorate in the United Kingdom. I don’t know whether the same data relates to the United States but it quite possibly does.
Some sort of reaction is required otherwise the vaccine will be ineffective. This is because a vaccine stimulates the immune system. Although what is classified as a “vaccine reaction” is not defined, a low-grade feeling of being unwell (malaise) occurs not uncommonly. As an estimate, about 3% of vaccinations result in significant reactions. Sturgess defines this as a reaction that lasts for more than 24 hours and is low-grade, meaning feeling unwell.
Therefore for the vast majority of cats the reaction is short lived. The critical issue is vaccine-associated sarcomas otherwise described as “invasive soft tissue tumours”. These develop at the site of the vaccination which is usually in the neck region of the cat. This reaction has been calculated to be 1 in 10,000 or 0.01%. This is based on a study in 1999 by Hendrick. I think this bit of information is useful to concerned cat caretakers because there is a lot of discussion about whether to risk a vaccination or not. The risk of developing a cancer is very slight.
However, as a result of the critical nature of a vaccine-associated sarcoma, the American Association of Feline Practitioners recommended that vaccines should be given to the “distal limbs” of the cat. A distal limb is one that is farthest away from the point of attachment so that if an amputation has to take place the cat suffers the minimum loss. In addition, different vaccines are given at different sites on different limbs in order to attempt to identify vaccines which are associated with the development of sarcomas.
Associated: Cat Vaccine Recommendations.