why do rabbi shots have to be injected through the abdominal??

Question

At one time, the rabies vaccine (and by the way, rabies is the only disease in which the vaccine is administered post-exposure) required slow absorption and prolonged activity, so it needed to be injected in an area with a large amount of subcutaneous fat. In humans, that area is the abdomen (the upper arm and buttocks are more muscle than fat). Today’s rabies vaccine is a series of five shots given in the upper arm over the course of one month. In the United States, the most likely carriers of the disease in the wild are foxes, raccoons, skunks and bats. Rodents and lagomorphs (hamsters, squirrels, rabbits, chipmunks, etc.) have rarely tested positive for rabies.

Source (not a great site):
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