The Marek’s vaccine is there for those who have a MD positive flock. Chickens shouldn’t be vaccinated unless you have a positive for said disease in your flock or area. Most vaccines make birds carriers of whatever you vaccinated them for. Poultry aren’t like dogs, cats or humans, they don’t need yearly vaccines or any at all. I’ve read that MD is only so deadly because of vaccines. If people didn’t vaccinate all Willy nilly, then we wouldn’t have so many cases of MD or other diseases. Just my 2 cents from my research.