It doesn't need to be 100% effective in humans, it just needs to slow transmission enough such that the rate of incidence declines in the direction of eradication.
It will have the opposite effect, if people who get the vaccine start behaving as though they are 100% protected (which, many people will).
It will have the opposite effect, if people who get the vaccine start behaving as though they are 100% protected (which, many people will).