Veterinarian-written / veterinarian-approved articles for your cat.

Vaccine-Associated Sarcomas in Cats

cat_eyesFeline vaccine-associated sarcomas, or injection-site sarcomas, are an uncommon but devastating consequence of vaccination in cats.

What Are Feline Sarcomas?

Sarcomas are a specific type of soft-tissue cancer. Soft tissues include the non-bony and supportive connective tissues in the body. Research has firmly established a causal relationship between feline vaccination and sarcoma development, and tumors have been consistently identified at sites of previous vaccination. The presence of such tumors has established necessary evaluation of vaccine protocols, and former vaccination schedules have been re-analyzed. A professional organization, the Vaccine-Associated Feline Sarcoma Task Force (VAFSTF), was established in 1996 to address this issue. VAFSTF has formed comprehensive panels to assess risk factors, diagnosis, treatment, and prevention of feline sarcomas since its inception.

What Triggers Vaccine-Associated Sarcomas?

The cells in vaccine-associated sarcoma sites demonstrate signs of inflammation similar to post-vaccination inflammatory reactions. In addition, portions of aluminum adjuvant have been detected in tumors. This suggests that intense inflammation could be triggered by adjuvant administration, leading to extensive overgrowth of connective tissue cells called fibroblasts and possible transformation into tumor cells. Adjuvants are substances which are added to vaccines to create robust immune responses; many common veterinary adjuvants are aluminum-based.

Individual feline genetics could also play a role in development of the tumors.

Vaccines Most Commonly Associated with Fibrosarcoma Development in Cats

Feline leukemia virus (FeLV) and rabies vaccines have been most frequently associated with vaccine-associated sarcomas, possibly because both were originally manufactured as killed vaccines mixed with adjuvants. It is theorized that vaccine and adjuvant are both deposited under the skin, triggering intense inflammation and ultimately resulting in sarcoma formation. These sarcomas are frequently identified in cats with a history of multiple vaccinations and are usually detected at sites of vaccination (historically administered between shoulder blades on cats).

How Common Are Vaccine-Associated Sarcomas?

The incidence of vaccine-associated fibrosarcomas is estimated at 1 case in 1,000 to 10,000 vaccines administered. Vaccine-associated sarcomas are particularly devastating for owners despite the low incidence because they occur due to routine and recommended preventative medicine. It is theorized that sarcomas develop from weeks to years post-vaccination. Dull needles should be avoided to minimize inflammation. In addition, only single dose vaccine vials should be used to minimize bacterial contamination and additional inflammation.

Characteristics of Vaccine-Associated Sarcomas in Cats

Vaccine-associated masses in cats are highly aggressive and require multi-modal therapy including surgery, radiation therapy, and possibly chemotherapy. An owner who identifies a lump on their cat should immediately contact their veterinarian, particularly if lumps are identified at vaccination sites. The veterinarian will likely perform a fine-needle aspiration procedure, where cells from the lump are collected with a needle and viewed on a slide to try to diagnose the tumor.

Surgical removal is the cornerstone of therapy and provides a biopsy analysis and definitive diagnosis. The tumors need to be removed with wide margins; essentially, wide areas of tissue surrounding the mass on all sides and depths must be removed to confirm removal of all cancer cells. Other therapeutic methods (radiation and chemotherapy) are used as well. Your veterinarian can provide you with more specific details.

Prevention of Vaccine-Associated Sarcomas in Cats

VAFSTF recommendations strongly support measures to prevent vaccine-associated sarcomas. These measures include the use of non-adjuvanted vaccinations, sharp needles and single-use vials, decreased vaccination frequency, and new vaccination sites. Adjuvanted vaccines should be administered on the rear limbs at the lowest possible (or distal) points. This can facilitate amputation of the entire mass and effective removal in case of sarcoma development. In clinical practice, FeLV vaccines are usually given in the left rear limb and rabies in the right rear limb. The non-adjuvanted feline viral rhinotracheitis/calicivirus/panleukopenia (FVRCP) vaccine can be administered in the right front limb. Non-adjuvanted products are now available for both FeLV and rabies vaccines, and new studies have supported the use of many 3-year feline vaccinations.

Recent studies have suggested that vaccine-associated sarcomas located between the shoulder blades have decreased after VAFSTF recommendations; however, tumors on the side of the abdomen near the rear legs have increased, suggesting that vaccinations meant for the rear legs may have been erroneously administered in the abdomen. This strongly suggests that veterinarians must prioritize vaccine administration as low as possible on the rear legs to facilitate amputation if necessary.

It is the fervent hope of cat owners and veterinarians to collectively prevent all future cases of vaccine-associated sarcomas. There has been much progress in the general understanding of these tumors, and efforts must continue to try to minimize them.

You May Also Like These Articles:

Rabies in Cats

Feline Leukemia Virus

FIV: Feline Immunodeficiency Virus

FIP: Feline Infectious Peritonitis in Cats

Worms in Cats: Feline Intestinal Parasites

How to Be Prepared for Your Cat\'s Veterinary Bills

Disclaimer: This website is not intended to replace professional consultation, diagnosis, or treatment by a licensed veterinarian. If you require any veterinary related advice, contact your veterinarian promptly. Information at is exclusively of a general reference nature. Do not disregard veterinary advice or delay treatment as a result of accessing information at this site. Just Answer is an external service not affiliated with

Notice: Ask-a-Vet is an affiliated service for those who wish to speak with a veterinary professional about their pet's specific condition. Initially, a bot will ask questions to determine the general nature of your concern. Then, you will be transferred to a human. There is a charge for the service if you choose to connect to a veterinarian. Ask-a-Vet is not manned by the staff or owners of, and the advice given should not delay or replace a visit to your veterinarian.