Would a single cortisone injection in the knee given two weeks prior to receiving post exposure rabies vaccine decrease the efficacy of the vaccine ?

GoodThinking because: all steroids ↓ immune system response & the whole point of using a vaccine to stimulate a new or ↑ed immune response to a small specific set of stimuli (antigens) to enhance self-protection, ideally a week or more (time for initial, or secondary, immune response to develope) prior to exposure. Answer depends on steroid & route given. In most cases likely little to no difference after 2 wks.

Unlikely: There are not a lot of studies on this topic, but I would say that decreased efficacy is rather unlikely in this scenario.