We have had the same question recently as we are getting ready to hatch more eggs. We decided to vaccinate. Mareks is so common in many areas, and the vaccine is a one-time deal for day old chicks. I think it's worth it. I have had several friends lose birds to this terrible disease.. it's a flock killer, and they didn't show or anything - just had backyard birds. It's a horrible death for the bird. Sorry I can't give a definite answer, but maybe our opinion will help.