With ds1, we had an outbreak of measles in this town last year, so he was 3.5 when he had his. I told him he was going to Woolworths to choose some sweets, then a scratch on the arm, then he would get to eat all his sweets.
This wasn't conditional on him being brave. He gets them anyway, even if he had screamed, cried, lashed out, whatever, it's not a reward to me, it's a fact - you go for injections, you get treats.
I may be doing a bad thing, I don't know, but I felt that the anticipation of a big bag of chosen sweets that he knows are coming would stop the fear. It seemed to work, but I don't really know, because his dad took him. I have a needle phobia, so I care deeply about him not being frightened. I wanted the sweets to overshadow the jab. It seemed to work very well, he didn't even cry and his dad said his eyes never left the pic'n'mix bag!