Vaccines and Tumor Formation in Cats

This is a subject which has come to the forefront in the last few years. It concerns the increase in the formation of a nasty variation of a tumor known as fibrosarcoma. It is so different from the usual fibrosarcoma that it is being called “vaccine-induced sarcoma” or “injection induced sarcoma”.

It has been noticed on some older cats that a tumor may form in the area where vaccines and other injections have been repeatedly given, usually in the shoulder blade area. The largest proportion of injections given in that area are vaccines. It seems linked to the arrival ten to fifteen years ago of the FeLV and rabies vaccines, both of which are given subcutaneously. They are dead virus vaccines, with a substance, called an adjuvant added to the vaccine. The adjuvant stimulates the immune system to produce a stronger response.

It is felt that the repeated injections of the vaccines with the adjuvants into one area may be somehow over-irritating the tissue causing tumor formation with time. An obvious solution is to give the vaccines in separate areas. We are currently doing this for all cats. The FVRCP (feline distemper and upper respiratory vaccine) is a modified live virus vaccine and will continue to be given in the shoulder blade area, as it has been given for decades. The FeLV vaccine will be given in the LEFT rear leg, under the skin. The rabies vaccine will be given under the skin in the RIGHT rear leg. This separates the vaccine by distance on the body, avoiding the concentration of irritation in one area.

Another possible solution is to separate the vaccines by time; give the vaccines on separate days. The question is “How long do you wait between vaccines?” If you chose to separate by time as well as location, we would recommend two to three weeks separation, with the FVRCP given first, the FeLV given second, and the rabies, if needed, given third. Most people will just give all the vaccines due on the one visit for convenience sake. It is felt that the physical separation is the most important factor.

Experts are recommending giving adult cats each vaccine every three years. The FVRCP would be given one year, the FeLV vaccine the next, and the rabies the third year. Kittens receiving their first set of vaccines will continue to receive vaccines using the usual protocol and will have the vaccines separated by location on the body.

Please report any side effects such as fever, lethargy, decreased appetite, swelling, or pain at the injection site. Some swellings or lumps may take several weeks to form. These are especially important to report, as we may recommend discontinuing the vaccine that caused it.

These recommendations are based on what we know at this time and should be individualized for each situation. It comes down to risk versus benefit. Right now for most cats, the risk from disease is still greater than the risk from the vaccine; so most cats should get the vaccines. However this not true for ALL cats. Be sure to discuss this with us so that the best decision can be made for your cat.


  • Under New Ownership
  • Communication & Compassion are Our Goals
  • We Give You Options For Your Pet’s Health Needs.
  • We Educate You So You Can Make an Informed Choice.