All birth control methods have a risk of failure, but vasectomy is the most reliable of all the methods available. According to the UK national sterilisation guidelines (2004), the failure rate of vasectomy should be quoted as approximately 1 in 2000 (0.05%) after clearance has been given.1 Therefore the chances of a vasectomy spontaneously reversing itself are actually very rare. According to Harvard Medical School, "Vasectomy can reverse itself, but it is a very rare event. It develops in only about .025% or one in 4,000 vasectomies".2
It should be appreciated that as late recanalization is in fact very rare, the research into it's exact causes is limited by the small number of subjects being available at the time any particular study is being undertaken. Very often the published literature is in the form of individual case histories, as opposed to larger studies.