Serious side effects have also been reported as a result of FeLV vaccination; in particular, a small percentage of cats who received FeLV vaccines subsequently developed vaccine-associated sarcomas, an aggressive tumour, at the injection site.
Efek samping yang serius juga telah dilaporkan sebagai hasil dari vaksinasi FeLV; khususnya, sebagian kecil dari kucing yang menerima vaksin FeLV kemudian dikembangkan vaksin terkait sarkoma, tumor agresif, pada daerah suntikan.