A vaccine-associated sarcoma (VAS) is a type of malignant tumor found in cats (and rarely, dogs and ferrets) that has been linked to certain vaccines. VAS has become a concern for veterinarians and cat owners alike and has resulted in changes in recommended vaccine protocols. These tumors have been most commonly associated with rabies and feline leukemia virus vaccines, but other vaccines and injected medications have also been implicated.
Vaccine-associated feline sarcoma or injection site sarcoma -- is prompting veterinary experts to advise cat owners and their veterinarians to think twice about whether all vaccinations are necessary for all cats. The risk of developing a sarcoma increases as the number of vaccines given in one area increases.