I'm posting a comment I put on the "He said, she said" thread, b/c I thought it was pertinent to your question.
I don't vaccinate, don't intend to. From the Mareks research I've done: Vaccine only masks the disease in that the lethal tumors don't show up. Vaccinated poultry can catch it, can spread it. So, you may have vaccinated your first flock, and continued with a closed flock, only to still end up with it infecting your flock (chicks you've hatched yourself), and you may never know that the disease came from one of your vaccinated birds. Also, it is most likely to show up in over crowded conditions and in immune compromised birds. Well tended poultry with healthy immune systems are not likely to become ill with it. (I'm not saying they WONT, but I'm saying it's less likely to have it show up in a home flock. The commercial folks vaccinate b/c their flocks are over crowded, stressed, living in filth, and immune compromised. Furthermore, turkeys carry a strain of Mareks, which is less lethal to chickens. So, if you have a population of turkeys around your flock (I have tons of wild turkeys), your chickies are most likely to pick up that strain, which will afford them immunity to the more lethal strains. (Similar to the milk maids of the past being immune to small pox b/c they'd been infected with cow pox.) As far as other diseases, I have similar opinion: it's a shot in the dark. Money spent for a disease that may never be encountered, and if your flock is healthy, they will most likely not get sick.
How to build flock immunity? I'm a fan of getting chicks exposed to native soils while their "peri-hatch" immunity is highest. (within the first 2 weeks) I give them a plug of sod: toss it right into the shavings in the brooder (upside down). I also put them on fermented feed. If you don't want to do FF, then you can do natural ACV with the mother, plus the sod, and add some packaged probiotics.
______________________________________________________________________________________________________________________________
Additionally, once you've received your chicks, the vaccination options are much more difficult and expensive. The time to make that decision is when ordering your chicks.
Regarding medicated feed: It contains Amprolium, which is a Thiamine blocker. This prevents thiamine uptake by the coccidiosis organism, therefore it can't replicate to get a strong foot hold in your chicks gut. Cocci are present in all soils. They cause disease when chicks are immune compromised, stressed, or otherwise weakened, kept in a damp environment where the cocci thrive. You can have chicks get coccidiosis without ever being exposed to the soil, or you can raise chicks for a whole life time without ever encountering it. I've never had it in any of the chicks I've raised. I've never given them medicated feed, and never intend to b/c I will use the natural approach to boost their immunity. It's an individual decision regarding medicated feed, and neither approach is wrong. What is wrong is if some one tells you that what ever choice you make is wrong.
