Anyone who has had a pet die from a preventable disease can tell you about the benefits of vaccinating your dog or cat. Unfortunately, vaccinating your pet can sometimes be harmful. Vaccines can cause side effects, such as a lump at the injection site, reduced appetite, fever, vomiting, or encephalitis (inflammation of the brain). Some cats develop cancerous tumors at the sites of rabies or feline leukemia vaccinations. Do you vaccinate your dog or cat for rabies. Distemper. Feline leukemia. If you do, how often? If you don’t, why not? The answers to these questions are less clear-cut than they were a decade ago.