Why do i have to wait a few weeks after getting a cortisone shot to receive the shingles vaccine?

Wouldn't be effectiv: A cortisone shot has many benefits but one side effect can be to shut down part of your immune recognition system. A shot within 3-4 weeks may not give you any benefit, while waiting would allow your immune system to wake up and be aware of the stimulus. That is what makes the shot worthwhile.

Steroids: Steroids reduce the body's ability to make antibodies and fight off 'foreign' invaders. A vaccine is trying to stimulate the immune system to make antibodies by presenting as 'foreign'. The Cortisone shot would make the vaccine ineffective if given at the same time.

You really don't: If your immune system is compromised by disease (ex. cancer) or treatments (ex. chemotherapy) the shingles vaccine will likely be less effective or disease could be triggered. Steroid medication, such as cortisone, can suppress the immune system. To do this the cortisone has to be systemic (usually pills) and of at least two weeks duration. A shot shouldn't suppress the immune system. Vaccine ok