Among the conclusions reached by researchers following the study were:

• No vaccine is free of sarcoma risk.
• Other injectable drugs could potentially initiate sarcomas in felines, including long-acting penicillin and long-acting corticosteroids such as dexamethasone and methylprednisolone.
• The results of the study can be considered reassuring because “the use of medically indicated products does not appear to be associated with substantial morbidity of sarcomas, and their propensity to cause tumors appears to be small.”

Details of the study
The research team began by dividing cats into three groups: cats with soft tissue sarcomas (cases), cats with tumors at non-vaccine regions (control group I), and cats with basal cell tumors (control group II). They also specified that they were comparing recombinant to inactivated and MLV to inactivated vaccines, as well as the use of other injectable products.

Researchers used a questionnaire distributed to the cats’ veterinarians to document demographic, sarcoma location, basal cell tumor, vaccine, and other injectable history data. The questionnaire required veterinarians to submit detailed vaccine and injection histories, making use of a body map grid to indicate the region of injection sites. This approach was intended to achieve the goal of “temporally and spatially refining definitions of cases and vaccine and injection exposures.”

After examining all data collected during the study, the researchers concluded that there is a connection between the administering of certain vaccines (recombinant vs. inactivated rabies) and other injectable materials (e.g., long-acting corticosteroids) with sarcoma development.

According to the study’s authors, there hasn’t existed a definitive method of determining the risk levels of each type of vaccine or injectable substance. By conducting further studies to assess the risk factors of IJS, researchers note that it could lead to the development of practices that have a reduced risk of inducing IJS.