What is an adjuvant, anyway?

Killed virus does not stimulate the immune system as well as modified live virus but there are exceptions to this rule. With rabies it is simply too dangerous to risk any problems with the virus modification process and most rabies vaccines use killed virus for this reason.   With more complex immunosuppressive viruses such as the feline leukemia virus, live virus serves to decrease the immune response, not increase it. For feline leukemia virus vaccination, killed virus is used.

To facilitate immune stimulation with a killed virus, a substance called an "adjuvant" is used. This material holds the virus in the area of the vaccination for a couple of weeks so it can be released slowly, allowing immune stimulation to take place over a longer time period. This kind of stimulation can lead to some local inflammation in the area of the vaccination and one theory is that this inflammation is what leads to precancerous changes in the local cells.  Indeed, some fibrosarcomas have been found to have vaccine adjuvant embedded within them.