I understood Merek's vaccination was best when they were a day old.
I had the hatchery vaccinate my chicks that I purchased. UC Davis recommends Mereks vaccination for all flocks, large and small. I have heard of small backyard flocks around SoCal that have had it.
However, we did an experiment with hatching eggs from the grocery store 10 months ago, and our chick from it did not get vaccinated since she hatched at home. So far she has no health problems. The rest of the flock and neighborhood chickens were vaccinated. If I could of, I would have vaccinated the one chicken that wasn't vaccinated after listening to how devastating Mereks can be on a flock.
*Medicated Feed only prevents coccidiosis. If you don't have chicks vaccinated against cocci, folks give their chicks medicated feed instead. It isn't intended to be used to treat anything else. It isn't an antibiotic or antiviral; it inhibits the growth of the parasitic protozoa that causes coccidiosis.