The benefits of vaccination greatly outweigh possible risks. Following vaccination your cat may experience mild and short-lived reactions (malaise), such as poor appetite, lethargy, and fever that resolve without treatment. Any symptoms that persist for more than a day or two should be discussed with your veterinarian. Rarely, more serious allergic reactions occur and may include vomiting, diarrhea, facial swelling, or difficulty breathing. These serious reactions appear within minutes or hours of vaccination and require immediate veterinary care. Another uncommon reaction is a tumor at the injection site that develops months or years after vaccination. Talk to your veterinarian about any persistent lumps or swellings at injection sites.