Vaccine-associated sarcoma (injection site sarcoma, vaxosarcoma or vaccine site sarcoma) is a rare but aggressive type of cancer which is linked to vaccinations, especially rabies, and feline leukemia vaccines. 
VAS was first noted in the late 1980’s and a link was made between certain vaccines and VAS in 1991. Dr. Mattie Hendrick, a veterinary pathologist at the University of Pennsylvania School of Veterinary Medicine observed sarcomas appearing at the site of vaccinations. Prior to this in 1985 several changes to vaccinations had occurred.
The killed rabies vaccine replaced the modified live virus with the addition of adjuvants to stimulate a stronger immune response.
Introduction of the FeLV vaccine.
Pennsylvania made the rabies compulsory in 1987.
In 1987 the rabies vaccine became compulsory in Pennsylvania.
The exact incidence is in the range of 1 – 10 cats per 10,000 who receive the rabies or FeLV vaccine.  Rabies and FeLV are most common vaccines associated with injection site sarcoma, however, ther vaccines and non-vaccine products have also been reported on occasion.
The cause is still not fully understood, the adjuvant holds the antigens at the vaccine site which is released over a period of time, to stimulate an immune response. It is speculated that this can cause inflammation, which in turn develops into cancer.
The most common sarcoma is fibrosarcoma, although there are reports of other types of sarcoma.
The most common symptom is a firm subcutaneous swelling at the site of the injection, there may also be ulceration. If you notice a lump at the site of a recent injection, see your veterinarian.
Your veterinarian will perform a complete physical examination and obtain a medical history from you. A biopsy will be taken which will be sent to a laboratory for examination.
Aggressive action is necessary if the following criteria are met:
The lump is larger than 2cms
The lump persists for more than 3 months after vaccination
Any lump which continues to grow one month after vaccination
CT scan and X-rays to determine the extent of the lesion and see if the cancer has spread (metastasised).
Vaccine-associated sarcomas are fast growing and very invasive. Removal involves a wide margin or amputation of the limb.
Radiation therapy will be a follow-up or occasionally chemotherapy.
VAS is a serious issue but we must remember that vaccinations have saved millions of lives. Vaccinations are still an important aspect of cat health. Your veterinarian is the best person to advise you on vaccinations and you should speak to him about any concerns you may have. References: