It is well known in the veterinary community that an aggressive tumor (fibrosarcoma) may result from any vaccine at its injection site. Because of this veterinarians have standardized placement of each commonly-used vaccine to track the incidence of this cancer in each vaccine.
It is most seen in leukemia and rabies vaccines. Incidences range from 1 in 10,000 to 1 in 100,000. This risk seems minimal, but enough cats (including one of mine) have developed this largely untreatable tumor and have died from it, to have created much concern and many reports in the veterinary literature.
Please discuss judicious use of vaccines with your veterinarian. Current guidelines on vaccines from the American Association of Feline Practitioners are on their website. Also, educate yourself about which vaccines are required by your county. In the Portland, Oregon area, only Multnomah County requires a rabies vaccine in cats.
Laura Speirs, Feline Behavior Consultant