As with a vasectomy, you will not be sterile immediately after the valves are closed. The reason for this is the biology of the male reproductive system, which will still contain sperm cells for a certain amount of time after the connection to the testicles is blocked. There will still be sperm cells present in the semen for up to three months or until about your 30th ejaculation. Getting a spermiogram after three months will offer more certainty.