Such persistent reaction could be a sign of a type of cancer called feline injection site sarcoma (FISS). (www2.vet.cornell.edu)
Feline Veterinary Emergency Assistance — Provides emergency financial assistance to cat and kitten guardians who are unable to afford veterinary services to save their companion with Vaccine Associated Sarcoma (VAS), also referred o as Injection Site Sarcoma (ISS). (nokillsouthcarolina.org)
Comparative vaccine - specific and other injectable - specific risks of injection site sarcomas in cats. (suffieldvet.com)