Something like two-thirds of men who undergo reversal of a vasectomy have at least one of their ductus deferens (the tubes that transport the sperm from testicle to penis) successfully sewn back together and can therefore ejaculate sperm afterwards. Sometimes too much tube has been removed and the operation is impossible. Sometimes, even though the surgeon thinks he or she has re-sewn the tubes, sperm still cannot get through.
Sperm counts are never as good after a reversal of vasectomy as they were before the vasectomy.
Perhaps the greatest barrier to fertility after vasectomy reversal is the presence of anti-sperm antibodies. These are common, especially in those men who had their vasectomies many years previously. They can inhibit the sperm from moving or they can prevent fertilisation or both. Even if a good sperm concentration is achieved after a vasectomy reversal, anti-sperm antibodies can prevent pregnancy.
Surgical sperm recovery and IVF/ICSI is now a reasonable alternative to vasectomy reversal and is not affected by anti-sperm antibodies.