Feline vaccination side effects

Updated April 17, 2017

Feline vaccinations can protect your cat against many harmful diseases, including rabies, feline leukaemia and the feline immunodeficiency virus, but they have the potential to create side effects—sometimes serious. Many side effects will resolve on their own, but there are a few that can become life-threatening if not treated. Consult your veterinarian if you are concerned about any vaccine side effects your cat experiences.

Decrease in Appetite and Activity

Dr. Holly Nash of the Doctors Foster and Smith website reports that some cats will experience mild side effects after receiving vaccinations. Common side effects include fever, lethargy and a decrease in appetite. These symptoms typically last a day or two after the vaccination is administered and usually resolve without any medications. Cats that receive certain Chlamydia vaccines may present these symptoms, along with weakness and lameness, for a few weeks after the vaccination has been given. Some cats require medications such as corticosteroids to alleviate their symptoms.

Discomfort and Swelling at Injection Site

Sometimes cats experience soreness and pain at the injection site. This area may become red and irritated and the cat may limp, depending on where the injection was given. The Cornell College of Veterinary Medicine claims that some cats will develop small bumps or swellings where the injection was given. The swellings are usually painless and go away on their own after a week, but owners should call their veterinarians if this symptom occurs. If the cat is in severe pain, a veterinarian may prescribe medication to relieve the pain.


Though rare, it's possible for a cat to develop a sarcoma—a type of cancer—where the injection was given. Sarcomas are tumours that invade deeply into underlying tissue. Vaccine-related sarcomas could be caused by abnormal responses to the adjuvant in vaccines, infections from other viruses or genetic predispositions. This type of reaction is extremely rare, as only one out of about 5,000 to 10,000 vaccinated cats will develop sarcomas. Some cats develop small, harmless lumps at the injection site, but if the lump persists for more than three months after the vaccination, grows in size, or is larger than two centimetres in diameter, consult your veterinarian as these could be signs of a sarcoma. Surgical removal of the tumour, along with chemotherapy or radiation, is often indicated.


Anaphylaxis is a rare, life-threatening allergic reaction to a vaccine. If left untreated, anaphylaxis can result in shock, cardiac failure and death. Cats that experience anaphylaxis will often have facial swelling, fast heartbeats, pale gums and cold limbs. Veterinarians typically treat this reaction with intravenous fluids and epinephrine.

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