Also commonly known as injection site sarcoma, vaxosarcoma or vaccine site sarcoma, injection site sarcoma is a rare but aggressive type of cancer which is linked to vaccinations, especially rabies, and feline leukemia vaccines. 
VAS were first observed in the late 1980’s and a link was made between the administration of certain vaccinations and vaccine-associated sarcoma in 1991 by Dr. Mattie Hendrick, a veterinary pathologist at the University of Pennsylvania School of Veterinary Medicine. She had observed sarcomas appearing at the site of vaccinations. Prior to this in 1985 several changes to vaccinations had occurred. Firstly, the modified live virus in the rabies vaccine was replaced with a killed rabies vaccine, given subcutaneously (under the skin). Adjuvants were added to help stimulate a better immune response. Around the same time, the FeLV vaccine was also introduced. In 1987 the rabies vaccine became compulsory in Pennsylvania.
The exact incidence is not known, but it is believed to be in the region of 1 – 10 cats per 10,000 vaccinated for rabies and FeLV.  Rabies and FeLV are most commonly associated with injection site sarcoma although other vaccines and non vaccine products have also been reported on occasion.
The cause is still not fully understood. The adjuvant is used to hold the antigens at the vaccine site and slowly released over a period of time, stimulating an immune response. It is speculated that this can sometimes lead to inflammation, which in turn may develop into cancer.
Fibrosarcomas are the most common sarcoma to develop although other sarcomas have also been reported.
What are the signs of VAS in cats?
The most common symptom is a firm subcutaneous swelling at the site of the injection, which may also be ulcerated. This should always be followed up with 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.
If the following criteria are met, aggressive action is necessary:
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
Once VAS has been confirmed, your veterinarian may also wish to perform CT and x-rays to determine the determine the extent of the lesion and see if the cancer has spread (metastasised).
Treating VAS in cats:
Vaccine-associated sarcomas are rapidly growing and extremely invasive. Removal involving a wide margin will be necessary or amputation if a limb is affected. This will be followed by radiation therapy and occasionally chemotherapy, although chemotherapy seems to be of little benefit.
While VAS is a serious issue it must be remembered 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.