I think is point is, from an individual's perspective, you'd still have to play it safe in the woods. People wouldn't be so inclined to get the vaccine if it didn't allow them to ignore the tick illnesses when they're out in nature. The vaccine wouldn't be so effective in that sense.
If there were a 100% effective vaccine against lyme's, I'd still check myself for ticks at the end of the day, and remove (and incinerate) any that I found. Ticks are nasty, whether or not they can give you diseases.
