It is possible for routine vaccinations to be harmful to cats. Fortunately, the likelihood of a serious or life-threatening reaction is rare. Some cats, especially kittens, may experience some loss of appetite, vomiting, lethargy, and/or muscle soreness. However, these signs should not last beyond 24 hours. Allergic or anaphylactic reactions can be life-threatening and need immediate attention, but I rarely see this happen. If your cat has had a previous reaction that lasted more than 24 hours, pretreatment with an antihistamine injection is oftentimes helpful.

Occasionally, a small lump can occur at a vaccine site. These are local inflammatory reactions that are usually not harmful and in most cases do not require treatment, but should be brought to my attention if detected. They usually resolve within 1-2 months. The current recommendation is to monitor these lumps closely and to biopsy any that persist beyond this time period.

The reason for this is that there has been some concern over a link between vaccine lumps and the subsequent formation of a type of cancer called fibroscarcoma. It is well known that fibroscarcoma can also occur spontaneously in cats at sites other than where vaccines have been given. Fortunately, the incidence of this cancer at vaccine sites is very low, perhaps as low as 1 in 10,000 vaccinated cats. Nonetheless, the veterinary profession is taking a close look at this. A task force committee has been formed to try and determine whether a cause and effect relationship actually exists.

My present recommendation is to vaccinate outdoor and indoor/outdoor cats against feline distemper/upper respiratory disease, rabies and feline leukemia virus because the risk to an unvaccinated cat from these life-threatening and fatal diseases far outweighs the unlikely occurrence of a tumor development or anaphylactic reaction. On the other hand, I don’t feel we should be over-vaccinating strictly-indoor cats. Indoor cats should receive a distemper/upper respiratory vaccine every three years but do not need the FeLV vaccine. An annual rabies vaccine regimen should still be encouraged for all cats due to public health implications unless a prior adverse reaction event has occurred. Please keep in mind that a yearly physical examination is good preventative health care for your feline regardless of which vaccines are provided.