How common having to get a second vasectomy after consistent positive semen checks?

My husband informed me he did not get a semen ysis after he had a vasectomy during his first marriage. That got me to wondering: How common do vasectomies fail and have to be re-done? I know the chances of pregnancy are 1 in 1000 with vasectomy but uming that's after it's proven there is virtually no sperm in subsequent semen ysis.