The incidence of vaccine‐associated sarcomas in cats: a systematic review and meta‐analysis

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Abstract
Objectives: Feline injection‐site sarcoma is a rare but serious adverse event associated with vaccination in cats. Published estimates of feline injection‐site sarcoma incidence vary widely, contributing to uncertainty in clinical decision‐making and incidence communication. Materials and Methods: We conducted a systematic review and meta‐analysis to estimate the incidence of feline injection‐site sarcoma following vaccination in cats and to assess the certainty of the evidence. MEDLINE ( via Ovid), CAB Abstracts and Web of Science were searched from database inception through January 2026 using terms related to cats, vaccination, sarcomas and adverse vaccine events. Eligible studies reported quantitative estimates of feline injection‐site sarcoma incidence, including incidence, incidence rate or measures of association. Two reviewers independently screened studies, extracted data and assessed study quality using the Joanna Briggs Institute checklist for prevalence studies. Incidence estimates were calculated using random‐effects meta‐analysis. Certainty of evidence was evaluated using the Grading of Recommendations Assessment, Development, and Evaluation framework. Results: Eight studies met inclusion criteria, representing 14,059,036 vaccinations and 411 reported feline injection‐site sarcoma cases. Study‐level incidence estimates ranged from near zero to substantially higher values per vaccination. The incidence was 8.8 feline injection‐site sarcoma cases per 100,000 vaccinations (95% CI 0.8 to 100.6), equivalent to approximately 1 feline injection‐site sarcoma per 10,000 vaccinations. Between‐study heterogeneity was very high ( I 2 = 99.61%), reflecting differences in study design, surveillance systems and reporting methods. Subgroup analyses comparing studies conducted before and after 2000 found no significant difference in feline injection‐site sarcoma incidence; although lower incidence estimates were observed in later studies, suggesting that the introduction of non‐adjuvanted vaccines may have reduced feline injection‐site sarcoma incidence. Overall certainty of evidence was rated as very low due to high risk of bias and imprecision. Clinical Significance: Feline injection‐site sarcoma remains a rare adverse event associated with vaccination in cats. Current evidence supports a low absolute incidence and reinforces the importance of routine feline vaccination while underscoring the need for improved surveillance and higher quality epidemiologic studies.

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