Since you can only touch one conductor of a USB connector at a time (or really at all) unless you stick your tongue into it, then any mains power applied to it is going to be hard pressed to cause you anything besides an uncomfortable tingling.

Hold a live power cable while walking barefoot across wet grass and I doubt you'd live to tell the tail.

Ah, so you do remember that the current is going to have to flow through you, and that will need something a bit more conductive than pavement and shoes.