During the procedure, the vasa deferentia of a man are severed, and then tied/sealed in a manner such to prevent sperm from entering into the seminal stream (ejaculate). Vasectomies are usually performed in a physician's office or medical clinic. CDC research has estimated there is a probability of 11 failures per 1,000 procedures over 2 years; half of the failures occurred in the first three months after the vasectomy, and no failures occurred after 72 weeks.