Vaccine Adverse Reactions in Cats: Welfare and Risk Assessment

Vaccination is essential for feline health, but vaccine-associated adverse events including injection site sarcoma require balanced risk assessment to optimise welfare outcomes for individual cats.

Key Facts

Feline injection site sarcoma (FISS) occurs in approximately 1-3 per 10,000 injections
FISS is a highly aggressive tumour; median survival without treatment is less than 2 months
Modern vaccines using non-adjuvanted formulations and altered injection sites have reduced but not eliminated FISS risk
Mild post-vaccination reactions including lethargy and injection site swelling are common and self-limiting
WSAVA guidelines recommend injection sites that permit wide surgical margins if sarcoma develops

Welfare Considerations

The welfare calculus of vaccination must weigh the low but serious risk of FISS against the high risk of fatal disease including panleukopenia, herpesvirus, and calicivirus in unvaccinated cats. For the vast majority of cats, vaccination substantially improves welfare by preventing potentially fatal disease. FISS, when it occurs, causes profound welfare harm requiring radical surgery and often causes premature death. Using vaccines that minimise FISS risk, injecting in sites that allow adequate surgical margins, and monitoring injection sites for persistent swelling allows welfare-centred vaccination decision-making.

What You Can Do

Follow WSAVA vaccination guidelines on injection sites to enable adequate surgical margins if sarcoma develops
Monitor injection sites for any swelling persisting more than 3 weeks after vaccination
Discuss the vaccination protocol for your individual cat with your vet, considering risk factors and lifestyle
Maintain vaccination records and report any persistent injection site reactions to your veterinary practice