I am very sorry Marek's got into your flock. That is a horrible disease. I really feel bad for you. Somehow your flock was exposed after two years. There is no way of knowing how.
Some chickens seem to have a natural resistance to Marek's. Some strains of Marek's are stronger than others, it depends on what part of their body it attacks. Some flock are not hit that hard by Marek's, some can be hit really hard. On average about half the chickens are affected when they contract Mareks' but that is just an average. For some flocks it can be 0% all the way to 100%. I don't know what will happen with your flock or how long it has been exposed, but there is a reasonable chance if they have not developed the tumors that cause the problems they may never. But you never know for sure.
Once Marek's is in your flock all your chickens have it and can spread it, whether they show symptoms or not. Even if they are vaccinated they can catch it and spread it, they just should not show any symptoms.
There is a lot of misinformation out there about the Marek's vaccine. They do not use Chicken Marek's to make the vaccine, they use the Turkey Marek's virus. That means they cannot catch Marek's from the vaccine. The vaccine will not prevent them from catching it, it should stop them from developing the tumors that cause the problems.
It costs money for a hatchery to vaccinate them. Tractor Supply, Rural King, or other feed stores are not likely to pay to get them vaccinated unless they are reimbursed when they sell them. It is best to assume that any chicks you get from a feed store have not been vaccinated.
After they are vaccinated they need to be kept away from any exposure to Marek's for two or three weeks to give the vaccine time to take effect. This is another hardship. Your flock has Marek's. If you get vaccinated baby chicks from a hatchery you need to keep them separated from the others (maybe brood them in your house and do not take them outside for three weeks). Marek's can spread by dust or dander. You should not wear the same cloths to take care of your chicks that you wear to take care of your flock, you could spread it that way. After three weeks of quarantine they should be OK but like I said, a hardship. Perhaps you can find a neighbor to split an order that will brood them for three weeks before you pick them up.
