Routine physical examination is recommended prior to administration of vaccines to cats. Patients in good health are the most likely to respond well to vaccination. Patients whose health status is not normal need to be closely evaluated to determine their suitability for receiving a vaccine and the ability of their immune system to respond. Factors such as age, pre-existing illness, and alterations of immune status need to be assessed.

Recommendations for the age at which vaccines are administered are influenced by several factors: age-specific risks of infection, age-specific risks of complications, ability of the cat of a given age to respond to the vaccine(s), and potential interference with the immune response by passively transferred maternal antibody. The most common factor causing inadequate response to vaccination in young kittens is inactivation of vaccine antigen by passively acquired maternal antibody. For the majority of kittens, vaccinating at 8 and 12 weeks will result in a protective response.

There are exceptions to this 8-12-week rule of thumb. One is in a situation of assumed persistence of passive panleukopenia antibody in a kitten from a queen with a known or suspected high titer due to recent vaccination or exposure to street virus. In such a case, an additional vaccination against panleukopenia should be administered at 15-16 weeks of age. Another exception to standard protocol can occur in kittens born into an endemic upper respiratory environment. In such cases, using an intranasal vaccine at 1 to 2 drops per kitten as early as 2 to 4 weeks appears a safe and effective way to initiate protective response.

Cats with a major illness, debilitation, or a high fever should not be vaccinated unless there is a compelling reason. Certain conditions are considered precautions rather than true contraindications for vaccination. Examples include mild respiratory disease without fever, or mild gastrointestinal disease occurring within a few days prior to vaccination. When faced with these conditions, some clinicians may choose to administer vaccine if they believe the benefits outweigh the risks for the patient.

The decision to administer or delay vaccination because of concurrent or recent febrile illness depends on the severity of clinical signs and the etiology of the disease. In humans, vaccines can be administered to patients with minor illnesses such as mild upper-respiratory disease or diarrhea with or without low grade fever with no loss of efficacy. The panel concludes that, in the absence of information to the contrary, it would be reasonable to follow this same guideline for cats.

Vaccines cannot stimulate optimum immunity in the presence of conditions that significantly compromise the immune state. Such conditions include nutritional deficiencies, genetic immunodeficiency, systemic disease, concurrent drug therapy, and stress as a result of transportation, climate, or other environmental factors. In these situations, either the immune system may be unable to mount an adequate response or there may be an unexpected adverse event. Every effort should be made to protect the patient from the risk of exposure and return the patient to health prior to vaccination.

Malnourished patients suffer from nutritional deficiencies that can impair immune response to a vaccine antigen. Deficiencies in taurine; vitamins A, B, and E; selenium; and iron are associated with decreased antibody response to antigen. Nutritional deficiencies should be corrected before completing a vaccination series.

Immunosuppression, whether primary or acquired, can cause inadequate response to vaccines. Primary immunodeficiency in cats is rare, but exists in Chediak-Higashi Syndrome in Persians. Acquired immunodeficiency occurs with certain infectious agents or immunosuppressive drugs. Cytotoxic drugs, such as methotrexate, cyclophosphamide, and azathioprine cause significant immunosuppression. Chloramphenicol, dapsone, clindamycin, griseofulvin, nalidixic acid, and sulfamethoxypyridazine have been associated with bone marrow suppression, resulting in leukopenia and immunodeficiency. Whenever possible, these drugs should be discontinued prior to vaccination.

Vaccinating cats receiving corticosteroids therapy is controversial. Depending on dose and duration, corticosteroids may cause functional suppression of immunity, particularly of cell-mediated immunity. However, dogs vaccinated during steroid administration and challenged did not develop clinical disease. The human literature recommends that children receiving short-term (less than 2 weeks) low to moderate daily maintenance steroids or low to moderate long-term, alternate day treatment with steroids for a condition which is not associated with a compromised immune system can receive live virus vaccines. Children on high doses of corticosteroids may be immunosuppressed and should not receive live virus vaccines. Furthermore, immunization with KV vaccines, although safe, may not induce an adequate immune response if steroids have been administered over a long period. Concurrent use of corticosteroids at the time of vaccination should be avoided if practical, but apparently corticosteroids do not result in ineffective immunization if short-term low to moderate dose regimens are used.

Both feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections can cause immunosuppression in some cats. Cats with short-term FIV infection (less than 2 years) have been shown to mount protective immune responses to vaccination. Vaccines may be ineffective if the virus causes severe immunosuppression due to impaired T-lymphocyte function. CORE vaccines should be administered to healthy FeLV- and FIV-positive cats; vaccine intervals should be the same as for non-infected cats. NON-CORE vaccines should be critically assessed as to risk/benefit ratio to determine if they are necessary. The use of an FeLV vaccine that may stimulate FOCMA antibodies to protect against tumor development has been advocated in FeLV positive patients. There is not enough information available for the panel to make a recommendation. KV vaccines are preferred for immunocompromised patients because of the potential risk for vaccine-induced infections with MLV vaccines.

The actual risks associated with vaccination of pregnant cats are poorly documented. With very few exceptions the use of vaccine in pregnant animals has not been shown to be detrimental to either the queen or her kittens. However, any vaccine component that is infectious (MLV or avirulent live) has the theoretical risk of crossing the placenta, invading the fetus, replicating, and causing fetal damage. This effect has only been demonstrated with the use of MLV panleukopenia vaccine in seronegative queens leading to cerebellar hypoplasia in kittens. Therefore, the use of MLV panleukopenia vaccines in seronegative pregnant queens is contraindicated. In addition, a pregnant queen would have the same risks of experiencing an adverse event associated with vaccination as would a non-pregnant female. Due to the physiological demands of her pregnancy, this could make the event worse than expected and it could complicate treatment and recovery. The possible sequel of a serious adverse event in a pregnant animal could include the loss of the pregnancy. While the panel concluded that the risks of vaccinating pregnant queens are likely overstated and that there are circumstances when the benefits of vaccinating a pregnant queen outweigh the additional risks, the routine vaccination of pregnant cats should be avoided. If vaccination is deemed important, the use of KV vaccines is recommended.