It is often impossible to establish evidence for cause-and-effect relationships on the basis of case reports alone because temporal association alone does not necessarily indicate causation, and because multiple antigens and possibly other agents are often administered to the patient at the same time. Reporting adverse events by practitioners is extremely important to 1) document actual frequency of occurrence, 2) stimulate studies, 3) confirm causal association, and 4) identify risk factors for adverse events. When observed, possible adverse responses to vaccine should be reported to USP and the vaccine manufacturer. A list of toll free numbers to use when reporting suspected adverse events can be found in Appendix E.

Benign local reactions are common in cats and can be characterized by pain or swelling at the injection site. Stinging or discomfort at the vaccination site is generally associated with excessively high or low pH levels or osmolarity that varies significantly from the physiologic norm. Localized swelling at the site of vaccination may be the result of several vaccine components, the most likely of which may be adjuvants. While the majority of these responses will resolve without treatment in 2-4 weeks, those that persist for 6 weeks or longer should be evaluated by cytologic exam and/or biopsy.

With the use of an intranasal vaccine applied to conjunctiva and nasal passages, local adverse responses can occur. These responses include sneezing, coughing, conjunctivitis, and potential shedding of vaccine virus.

An association has been established epidemiologically in cats between vaccine administration and the subsequent development of sarcoma. These vaccine-associated tumors most frequently are recognized at the site of vaccine administration and usually occur several months after the most recent vaccination. Some epidemiological studies suggest that with increasing number of antigens administered simultaneously at the same site there is an increased risk of vaccine-associated tumor development. The etiology of these tumors is unknown and the type of vaccines and antigens administered vary. Based on epidemiological studies, the rate of tumor development approximates 1-3 per 10,000 vaccines administered. Site recommendations have been established by the AAFP and the Vaccine-Associated Feline Sarcoma Task Force for antigens commonly administered to cats. These site recommendations are designed to facilitate treatment and to further understanding about the potential causal link between vaccination and tumor development. They are not designed, in and of themselves, to decrease the risk of tumor development. At our present level of understanding of this very complex problem, it is clear that for the majority of cats the benefits of vaccination far outweigh the risk of tumor development.

Vaccine components can cause allergic reactions in some recipients. Commonly implicated components include: 1) vaccine antigen, 2) animal protein, 3) antibiotics, 4) adjuvants, 5) preservatives, and 6) stabilizers.. These reactions can be local or systemic and can include mild to severe anaphylaxis or anaphylactic-like responses (e.g., generalized urticaria or hives, wheezing, swelling of the mouth, throat, face and eyes, difficulty breathing, hypotension and shock). These reactions are relatively rare, but may be observed on occasion. Treatment for anaphylaxis may involve the use of corticosteroids, intravenous fluids, or antihistamines. In peracute anaphylaxis associated with hypotension, time is of the essence and intervention with epinephrine is indicated. Epinephrine available in 1:1000 w/v is diluted 1 part to 9 parts saline and administered IV to affect in 0.2 ml boluses. More commonly, allergic reactions in cats may be manifested by recurring vomiting and diarrhea with or without blood. Veterinarians should always consider that a report of recurring vomiting or diarrhea in a patient within a few hours of vaccination could be allergy-mediated and not just a stress response. Since allergic responses tend to worsen with time, early treatment is essential. Treatment for this condition may include corticosteroids, intravenous fluids, or antihistamines.

There are also systemic responses that are not allergy-mediated. These would include fever, lethargy, reluctance to move, etc. These signs are expected to occur to some degree in a sizable proportion of a vaccinated population. The severity can range from subclinical to severe. Most cases will resolve in a few days without treatment. Only rarely will antipyretics or other supportive care be needed.

Avirulent live Chlamydia vaccines may cause atypical reaction in about 3% of vaccinated cats. These reactions are often poorly recognized by the practitioner. The severity of the reaction may vary from cat to cat. The precise etiology of this response is not known. Reactions to the live Chlamydia component of vaccines include lethargy, depression, anorexia, lameness and fever 7 to 21 days after vaccination. Cats with this reaction usually respond well with corticosteroids or aspirin (81mg every 2 to 3 days). However, it is contingent on the veterinarian to accurately diagnose the reaction and implement appropriate therapy.

The rate of adverse reactions appears to in cats vaccinated with multiple antigens, regardless of whether they are administered in combination vaccines or simultaneously as single antigens. Therefore, if a patient has a history of adverse reactions to vaccine, and if more than one antigen - with the exception of FPV,FHV-1,FCV is needed, antigens should be given at intervals of three weeks or more.

Preventing infectious diseases in cats with previous adverse reactions to vaccines requires serious consideration. A cat that has experienced a potentially life-threatening reaction, such as anaphylaxis or tumor formation at the site of administration, should not, unless at high risk, receive additional parenterally administered vaccines for the remainder of its life. Since further vaccination is discouraged, every attempt should be made to prevent exposure to pathogenic organisms by keeping cats indoors and away from other cats that could be harboring infectious disease. Cases in which rabies vaccines are to be discontinued in which there is virtually no risk of exposure to rabies, bats or other animals that could harbor the disease, local regulatory agencies can be petitioned to waiver any rabies vaccination requirement for that individual animal.

In cases where risk of infection exists, continued vaccinations may be an acceptable alternative but should be given as infrequently as possible. Serum neutralizing titers can be used to help determine the need for revaccination. When vaccination is deemed necessary, clients need to be informed that significant risks of serious adverse events, including death, exist. There are a few ways to theoretically reduce risk to these patients. Use of vaccines with fewer antigens may help. If more than one vaccine is needed, separate vaccines should be given (3 weeks or more apart to prevent vaccine interference). Since MLV vaccines normally contain no adjuvant and less protein than KV vaccines, the use of MLV products may be less likely to trigger allergic responses in these patients. Consideration can be given to the use of vaccines with an approved alternate route of administration, e.g., intranasal.

When a cat that has experienced a previous serious allergic event is vaccinated, several precautions should be considered. Patients should be pre-treated with routine doses of antihistamines and/or corticosteroids. Placement of an intravenous catheter should be considered. Following vaccination, cats should be observed in the hospital for 30 - 60 minutes and by the clients at home for several hours. Clients should be warned that even with these precautions, the animal may experience life-threatening responses.