What About Adverse Events?
No injection or medication is without some degree of risk, but we continue to vaccinate because in most cases it is much smaller than the risk of the disease itself. The overall incidence of adverse reactions in cats is reported to be about half of one percent and usually mild and self-limiting: lethargy, transient fever, and local inflammation. Anaphylaxis and death are, fortunately, extremely rare: about 1 in every 10,000 vaccines.
Vaccine associated sarcomas, a slow-growing but locally aggressive cancer associated with injections, occurs with about the same rare frequency as anaphylactic reactions. For cats without a history of vaccine reactions, the risk of sarcoma is usually outweighed by the benefit of the core vaccines. Pet owners can minimize the impact of sarcomas by monitoring injection sites for swelling after vaccinations. Swellings should be biopsied if they are larger than 2 cm, persist longer than three months, or grow one month past the date of vaccination. When addressed early, surgery is often curative.