Some adjuvants have been implicated in injection site sarcomas, and it happened often enough that drug companies changed the protocol
for administering the vaccines. It used to be that
dogs and cats were vaccinated either in the scruff of the neck (for SubQ vaccines) and in the hip muscle (for IM vaccines). That has now changed, and all vaccines are given in the leg because if an injection site sarcoma develops, the leg can be amputated and the animal can be saved.