A vaccine-associated sarcoma (VAS) or feline injection-site sarcoma (FISS) is a type of malignant tumor found in cats (and, often, dogs and ferrets) which 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 sarcomas have been most commonly associated with rabies and feline leukemia virus vaccines, but other vaccines and injected medications have also been implicated.[1]