Feline vaccine-associated sarcomas, or injection-site sarcomas, are an uncommon but devastating consequence of vaccination in cats. 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. VAFSTF has formed comprehensive panels to assess risk factors, diagnosis, treatment, and prevention since its inception.

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.

Feline leukemia virus (FeLV) and rabies vaccines have been most frequently associated with vaccine-associated sarcomas as both were originally manufactured as killed vaccines mixed with adjuvants. It is theorized that vaccine and adjuvant are both deposited under the skin, trigger intense inflammation, and ultimately result 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). 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.

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