Injection site sarcoma, (vaccine associated 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, any vaccine, and in fact injection (such as microchip, or long-term antibiotics or corticosteroids) can cause an injection site sarcoma.
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 and 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.
Veterinarians have created the 3, 2, 1 rule as follows:
3: Present in any form for three months after vaccination
2: Larger or equal to 2 cm across
1: Present for one month after vaccination and fast-growing
If any of the above criteria are met, the veterinarian will perform a wedge biopsy, which will be evaluated by specialist laboratory (histopathology).
Other diagnostic workups:
CT scan, xray and ultrasound: Where indicated, diagnostic imaging can help the veterinarian to determine the extent of the lesion. It is also useful to perform chest and abdominal imaging determine if there is any sign of metastasis (the tumour spreading).
Baseline tests: Complete blood count, urinalysis and biochemical profile to evaluate the overall health of the cat.
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.