Marek's vaccine is usually done at 1 day of age and it is recommended for vaccinated chicks to be isolated from other birds to give the vaccine a chance to kick in. Marek's vaccine does NOT spread to Marek's to non-vaccinated chicks or chickens--the isolation is for the protection of the vaccinated chicks, not the non-vaccinated ones.
The virus is ubiquitous so it's basically a race to see if the vaccine or the wild-type virus gets to the chicks first. That's also why vaccinating older chicks is pretty much useless--they've already been exposed.
If you aren't losing lots of birds to Marek's, decide if you want the vaccine. If you have birds under 18 months or so dying, then see if your local Ag department or animal health department or veterinarian can do a necropsy to determine the cause. Marek's has many different presentations and if you aren't testing for it, you may just be missing it. Most vets don't see virus infected chickens because people will bring in a dog attack or HBC chicken, but a chicken with an upper respiratory infection gets well on it's own, gets dosed with every antibiotic at the feed store and gets better or dies. You can see a bit of the phenomena on the boards here. Stitches are more likely to mean a visit to the vet than sneezing. I'm not saying it's wrong or bad, I'm just saying vets see a patient profile that isn't necessarily representative of what's out there.
I have 5 other 3.5 month old chicks who are Merek's vaccinated. These new chick aren't vaccinated and I wonder if this is something I should worry about. I really want my BOs and have really wanted a golden laced Polish for about a month (even since I saw one!) How prevalent is Merek's disease in the New England area? I know many of you don't vaccinate and many do. So both views are valid here.
