With cat vaccines, there is always a risk for adverse reactions. However, the risks are small compared to the risks involved with not vaccinating your cat from deadly illnesses such as rabies.
Anaphylaxis
Anapylaxis is a rare but life-threatening reaction to vaccinations that occurs almost immediately, usually minutes but less than 24 hours after vaccination, causing shock, respiratory failure and cardiac failure. Anaphylaxis more commonly occurs where killed vaccines such as rabies and feline leukemia are used and occurs in one of every 15,000 vaccinations administered.
Symptoms include vomiting, diarrhea, seizures, shock, coma, pale gums, cold limbs, quick heart rate, weak pulse and possible facial swelling. To prevent death, epinephrine must be administered immediately.
Cats who have suffered anaphylaxis should receive subsequent vaccinations, if deemed imperative, at the veterinarian's office with a catheter in his veins, ready to administer treatment in case of shock. Cats should remain in the office for a few hours afterward for observation. If given before vaccination, some antihistamines can reduce risk.
Vaccine-Associated Sarcoma
Rarely, one in every 10,000 to 30,000 cats vaccinated, a vaccine reaction can cause a tumor to develop. These fibrosarcomas develop on the connective tissue, usually in the area where the vaccination is given. They are most often associated with the feline leukemia vaccination, which has resulted in veterinarians recommending that vaccine only if your cat is at high risk.
Usually a small area of swelling will appear after a vaccination, but if that persists for more than three months, consult your veterinarian.
If you are concerned about this risk, ask your veterinarian to administer vaccines in your cat's hip, where tumors are easier to remove than between the shoulder blades.
Other Severe Reactions
Neurological reactions are the most common reaction to vaccinations reported in both dogs and cats. Most common among these is cerebellar disease, which causes loss of motor coordination, swaying, strange gait, facial ticks and sensitivity to light. These can range from serious to mild. This is usually associated with kittens vaccinated at five weeks of age or younger with a modified live vaccine such as FVRCP.
Lameness can also occur in kittens vaccinated with a live modified vaccine such as FVRCP. The lameness may take up to three weeks to appear and can last three to four days. Depending on the severity, it may need to be treated with antibiotics and pain medication.
A pregnant cat should never be vaccinated as this can cause birth defects or severe infections in newborns.
Other Mild Reactions
In addition to a mild swelling or allergic reaction at the site of injection, some cats may develop mild symptoms after being vaccinated. Symptoms include mild fever, decreased appetite, depression or mild respiratory problems. These usually last for only a day or two except following the chlamydia vaccination, after which the symptoms may last one to three weeks. In some cases, medication may reduce symptoms, but these reactions usually don't require treatment.
In rare cases, vaccinations can cause severe, life-threatening reactions. However, the benefits far outweigh the risks. If you are concerned about over-vaccination, consult your veterinarian about alternatives such as titers, which measure your pets level of immunity.