The term Vaccine Associated Sarcoma (VAS) refers to cancers triggered by vaccinations. VAS has been called a disease of well-loved cats because it arises from our efforts to protect our beloved pets and keep them healthy. While VAS is generally considered to be a feline health issue, these cancers also affect ferrets and occasionally, dogs.

In 1987 Pennsylvania mandated that cats be vaccinated against rabies, and within a couple years researchers and clinicians at the University of Pennsylvania's veterinary medical school noticed an increase in sarcomas (malignant tumors) in cats, usually located where vaccinations had been administered. The discovery of microscopic traces of vaccine components in some tumors was the "smoking gun" that associated these cancers with vaccines, and in 1991 they published their findings in the veterinary literature. These cancers were given the name Vaccine Associated Sarcoma.

Some sources prefer the term Injection Site Sarcoma (ISS) to reflect the fact that other kinds of injections can trigger cancer. The experience of the Feline VAS Support group, whose members have experienced almost two thousand cases of these cancers over a period of more than a decade, indicates that Injection Site Sarcomas triggered by injections other than vaccines make up a tiny proportion of these cancers, just a small fraction of one percent.

More than a dozen different cancers have been identified as VAS, but the vast majority is fibrosarcoma, a very aggressive cancer that spreads locally by infiltrating the tissues surrounding the tumor with microscopic tendrils of malignant cells that are ready to grow into new tumors when the original one is removed. These tendrils make successful surgery very difficult and often impossible. Chemotherapy and especially radiation can be used to increase the chances of a successful surgical excision, but these therapies do not ensure success, and their cost puts them out of reach for many cat guardians.

In November of 1996 Vaccine-Associated Feline Sarcoma Task Force was formed as a joint effort of the American Association of Feline Practitioners, American Animal Hospital Association, American Veterinary Medical Association, and Veterinary Cancer. The Task Force coordinated research and clinical studies related to these cancers, and one outcome was the development of new feline vaccination protocols that called for cats to be vaccinated in the legs, so that the leg can be amputated at the first sign of fibrosarcoma.

Unfortunately, a recent study (Journal of the American Veterinary Medical Assn., February 1, 2009) revealed that many vets continue to vaccinate in the scruff, which is a bad place to have a tumor develop because of the difficulty of obtaining a complete surgical excision.

Another development during the 1980s led to continuing confusion about these cancers. That was the introduction of killed virus vaccines to replace the earlier live virus vaccines that sometimes caused the diseases they were supposed to protect against. Vaccines produce inflammation, an essential part of the immunization process; it "gets the attention of" the immune system and stimulates the production of antibodies to fight the pathogen represented by the vaccine. Some of the new vaccines didn't produce enough inflammation to work properly, so substances called adjuvants were added to increase inflammation. This led to a suspicion that these adjuvants might be carcinogenic, and non-adjuvenated vaccines might be safer (citation: Round Table.) Now oncologists believe that inflammation itself is the VAS trigger, and research has shown that adjuvenated vaccines are no more likely to trigger VAS than non-adjuvenated ones of the same type, presumably because non-adjuvenated vaccines produce plenty of inflammation without the help of adjuvants (JAVMA, November 1, 2003.)

We still often read and hear that non-adjuvenated vaccines are safer, and the persistence of this idea might be due to the introduction of a new type of vaccines that are neither live virus nor killed virus. These are recombinant DNA vaccines, and they do not feature adjuvants. While they're too new for the kind of long term studies that would prove them safer than the older vaccines, oncologists believe that they are probably safer - in fact, so much safer that they should be less likely to trigger cancer when administered annually than the older types of vaccines that can be administered tri-annually (VAS Task Force Round Table Discussion.) If these vaccines are really safer it's not because they don't contain adjuvants; it's because recombinant DNA vaccines produce less inflammation.

The frequency of VAS is unknown. Back when VAS was not widely known the VAS Task Force estimated that VAS afflicted one cat in 10,000. Today we frequently see an estimate of three cats in 10,000 and some estimate one to ten cats in 10,000. One veterinary oncologist has proposed a frequency of one in 500. The wide variance in estimates suggests that nobody really knows how frequently VAS strikes, but the longer VAS has been studied, the higher the estimates of its frequency have become. The VAS Support Group, which has about 1500 current members, has had at least a dozen members who had two VAS kitties and two members who had three cats stricken by VAS. The group has had another dozen or more members who had one cat diagnosed with VAS, but lost a previous cat to a cancer that looked a lot like the one that afflicted their "official" VAS cat.

Although the experience of the VAS Support Group shows that many VAS cats can be treated successfully and go on to long term survival, these cancers continue to have a high mortality rate. The only way to eliminate the threat of VAS is to stop vaccinating our cats, which would leave them in danger from a number of dangerous diseases, some of them (like rabies, for example) worse than cancer. It is possible to reduce the risk and to increase the chances for successful treatment when VAS strikes.

There are many kinds of feline vaccinations available, but not every cat needs all of them. Some of the diseases they protect against are dangerous to kittens but not to adults. Some of them are not present in some localities. Other factors, like the cat's lifestyle (indoor or outdoor, boarded or showed, and so on) can determine whether diseases present a real danger to an individual cat. Cat owners should consult with their veterinarians to determine which vaccines their cats really should have, with the goal of vaccinating only against diseases that present a real threat to their cats, and of vaccinating no more often than necessary. Veterinary oncologists also recommend that recombinant DNA vaccines, which produce less inflammation than other kinds of vaccines, be used when they are available (VAS Task Force Round Table Discussion.)

The best way to boost the odds of success in VAS treatment is to make sure that all vaccines are administered in the legs, as far down the leg as possible. That's because the scruff area, which is the traditional location for injections in cats, is a very difficult location to obtain a complete surgical excision of these aggressive tumors. The extensive experience of the Feline VAS Support Group suggests that amputees are much more likely to become long term VAS survivors than cats whose tumors developed in the scruff area or elsewhere on the body. As with any cancer, early detection makes successful treatment more likely, so guardians should always monitor their cats for suspicious lumps near vaccination sites. Since VAS has been reported to occur ten years or more after the triggering vaccination, this monitoring should continue for the life of the cat, and at the discovery of any lump, the cat should be examined by a veterinarian.

The literature on VAS is discouraging, often presenting these cancers as incurable and almost invariably lethal, but treatment can be successful. The VAS Support Group has had many long term survivors, cats that lived cancer free for seven or eight years or longer. Some continue to do well, extending the length of their survival, while others lived out their lives and died cancer-free following their treatment. If your cat is stricken, don't despair; treatment can be effective. While treatment can be expensive, financial assistance may be available.

If your cat is stricken by VAS, if your cat presents with a suspicious lump that might be VAS, if you are a veterinarian or technician with a professional interest in VAS, if you work with cats in any context as a professional or volunteer, please join the Feline VAS Support Group at

If your cat is facing amputation due to VAS, or for any reason, please visit this site that some of the VAS Support Group's members created to assist folks in that situation

For more information about VAS, VAS treatment, feline vaccines, financial assistance for treatment and more, please visit