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What are Tumors Related to Vaccinations?

The tumors develop from the reaction that happens in the cat's immune system. Commonly, the cancers that occur are fibrosarcoma, malignant fibrous histiocytoma, or myxosarcoma. These cancers can be very aggressive and can spread rapidly through the body, often becoming fatal. Current studies are underway to decrease the risk of vaccinations, but currently affected cats stand at 1 in 10,000 cases. It is important to note that no vaccine is 100% safe.

A very rare side effect of having your cat vaccinated is malignant (cancerous) tumor growth at the site of injection. These tumors are often referred to as Vaccine Associated Sarcomas (VAS) or Feline Injection-Site Sarcomas (FISS). Vaccinations are very important in combating life-threatening diseases, however, this rare but real condition must be considered. The tumors can develop in as little as two months after vaccine administration, or as late as 10 years after the injection. 

Symptoms of Tumors Related to Vaccinations in Cats

In most cases, the only symptom will be the tumor itself. If other symptoms have developed, the cancer has progressed to a life-threatening state. Veterinary care is vital at this point. Symptoms are as follows:

  • Lump under the skin where injection occurred, which may be growing
  • Lump that develops weeks after injection and does not dissipate
  • Swelling around the tumor
  • Skin ulceration at the site of the tumor
  • Limping or leg tenderness if tumor located on a limb
  • Loss of appetite
  • Weight loss
  • Lethargy

It is important to notify your vet of any lasting symptoms after vaccinations regardless of whether they are listed here.

Causes of Tumors Related to Vaccinations in Cats

While the exact cause of vaccine-associated sarcomas has not been determined, a few factors have been found to contribute to the issue. The tumors may form because of the adjuvants (ingredients added to increase the immune response in the cat) within the vaccine as opposed to the vaccination itself. All potential causes are listed below.

  • Adjuvants (especially those containing aluminum) 
  • The process of under-skin injection itself
  • Frequency of vaccinations
  • Inflammatory response of the cat

Diagnosis of Tumors Related to Vaccinations in Cats

The first part of this diagnosis will be to differentiate the vaccine associated sarcoma from other, more common vaccine reactions. Often a CT Scan or MRI will be requested to assess the tumor’s size and check for development of other tumors.

Biopsies may be performed on lumps greater than 2 cm. Incisional biopsies, in which a small amount of the tumor is removed for analysis, are preferred to excisional biopsies, in which the entire tumor is removed before diagnosis. A complete blood count will be requested, along with other blood work. A serum biochemistry panel (analysis of blood fluid) may also be performed. Urinalysis may be needed, along with lymph node aspiration, thoracic (upper and middle back) X-rays, or an abdominal sonogram. 

Treatment of Tumors Related to Vaccinations in Cats

Treatment of vaccine-associated sarcomas must be aggressive and fast. These tumors have a tendency to metastasize (spread) at a rapid rate. Treatment options are as follows:


Often, if the tumor is not far under the skin and located somewhere on the abdomen, removal surgery will be performed. Mesh reconstruction of the incision site may be needed to close the area where the tumor was taken from. The surgeon may mark the area with ink  in case further treatment is needed. New tumors may grow in the injection site as soon as 3-6 months post surgery. If the tumor if located on a limb, amputation is the safest course of action. Amputation is also related to longer survival times for cats with vaccine associated sarcomas. 

Radiation Therapy 

Many times, surgery is paired with radiation therapy for the best possible prognosis. Radiation destroys the microscopic portions of the disease that would be impossible to surgically remove. Often 16-19 treatments will run on a Monday to Friday schedule to complete the therapy. Skin and hair may be affected, and in some cases bone, muscle, nerve or organ tissue may divide. Radiation therapy can push the recurrence of tumors to 20 months or more. Radiation can also be used for palliative pain relief given on a daily basis to keep the cat comfortable. 


If there is a good chance that the cancer is already spreading, chemotherapy may be the best choice of treatment. Doxorubicin is usually prescribed and given four to six times with three-week intervals in between each administration. Some tissue damage is a common side effect. The kidneys may also weaken through treatment. Cats are generally dosed in a “quality of life” manner, meaning they tend to not be as sick from chemotherapy side effects as humans would be. 

Recovery of Tumors Related to Vaccinations in Cats

The prognosis for this condition varies greatly and worsens for every surgery needed. The earlier that detection and treatment take place, the better. Castrated male cats, cats who have already had a large tumor, cats who have had long treatment intervals, or whose excisions were incomplete tend to fare the worst. The tumors are likely to continue to recur in the injection site and metastization happens in approximately 25% of cases. Annual check-ups are a great way to monitor for any possible tumors before they become large enough to feel. 

To prevent these types of tumors occurring in the first place, vaccinations should be tailored to your cat’s individual lifestyle. Keeping a cat indoors reduces the number of vaccines the cat needs. Annual vaccines may often not be necessary. Blood titer tests can be used to see how many rabies antibodies are present in the cat to determine whether a new vaccination is needed or not. Non-adjuvanted vaccines can be used for both rabies and feline leukemia virus. A smaller dose of the vaccine may be used, and nasal or topical vaccines can sometimes replace the need for injections. Discuss all of these options with your veterinarian before subjecting your cat to a regular calendar-based vaccination schedule.