Although the administration of biological products is never entirely free of risk, currently available feline vaccines have an excellent safety record. That said, the true prevalence of adverse reactions is likely to be underestimated owing to underreporting by both veterinarians and owners.⁴⁹ Therefore, it is important to report any known or suspected negative events associated with vaccination. In the United States, veterinarians are requested to contact the manufacturer (Veterinary Technical Services) of the vaccine(s) considered to be involved. Veterinarians may also report known or suspected adverse events directly to the U.S. Department of Agriculture; the Center for Veterinary Biologics of the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service can be contacted by the following means:

• Visit USDA website
• Mail: Send the report form to the Center for Veterinary Biologics, 1920 Dayton Avenue, PO Box 844, Ames, Iowa 50010
• Telephone: (800) 752-6255.

At the time of vaccine administration, included in the patient’s permanent medical record should be the name, serial number, expiration date and manufacturer of the vaccine(s) given, date of administration, name of the person administering the vaccine(s), and the site and route of the vaccine administration. Adverse events should be recorded in a manner that will clearly alert all staff members during future visits.

Prevalence and Types of Adverse Reactions

Postvaccination adverse events in cats are considered rare.⁴⁹ In the most substantial survey to date, any adverse reactions were recorded for cats presented to Banfield Pet Hospitals in the United States between 2002 and 2005.⁴² During this period, more than 1.25 million doses of various vaccines were administered to nearly 500,000 cats. Adverse reactions within 30 days of vaccination were reported at a rate of 0.52% of cats vaccinated. The most commonly reported vaccine reactions are lethargy, anorexia and fever for a few days after vaccination, or local inflammation at the site of injection. ^42,50,51 In the Banfield Pet Hospital population, the risk of an adverse reaction was greatest in cats around 1 year of age and/or increased as the total volume of vaccine and number of vaccines administered concurrently increased.⁴²

Hypersensitivity Reactions

Although anaphylaxis (type I hypersensitivity reaction) is rare (approximately 1–5 per 10,000 vaccinations),^42,52 it may manifest as vomiting, diarrhea, respiratory distress, facial or generalized pruritus, facial swelling, and collapse.^51,53,54 Where revaccination is considered necessary in a cat that has experienced an allergic reaction, using a different vaccine formulation and premedicating with an antihistamine and glucocorticoid 20–30 minutes prior to vaccine administration is recommended, followed by close observation of the patient for several hours.^42,53 Other forms of hypersensitivity reactions (types II, III, and IV) almost certainly also occur in cats after vaccination, but these are rarely documented.

Postvaccination Monitoring

The Task Force recommends that veterinarians and owners monitor the vaccination site for swelling or lumps using the “3-2-1” rule. Biopsy of any mass present is warranted if it (1) remains present 3 months after vaccination, (2) is larger than 2 cm in diameter, or (3) is increasing in size 1 month after vaccination.^1,55 It is recommended to obtain an incisional biopsy on any masses meeting any of these criteria. Fine-needle aspirates may not provide diagnostic cellular tissue, whereas excisional biopsies rarely meet appropriate margins (5 cm in two fascial planes) as required in the case of injection-site sarcomas, thus increasing the morbidity and mortality risks associated with sarcoma invasion.