Marek's is a very common and very deadly disease. Whether you vaccinate or not depends on whether you want to take the risk. There's no way to know whether your flock will become infected or not. It's transmitted through wild birds so it's up to chance. Your chickens may be totally fine, or they may catch it and die. So you decide. If you have a large flock and don't mind losing some birds here and there, it's one thing, but if you have 5 pet chickens with names and your kids love them, losing them all to Marek's is a different picture. I personally don't like taking unnecessary risks, and don't like having my animals or people die of preventable diseases, so I vaccinate all my chicks (and kids). You can order the Marek's vaccine online (I get mine from Meyer Hatchery). It's not hard to administer - it comes with instructions, and you can find clear guides with pictures online, too. It doesn't have to happen right at hatch or at 1 day old or 2 days or whatever. What matters is that it needs to happen before the chicks are exposed to the outside world, where they can be infected before their immunity builds up. For the vaccine to be most effective, it needs to start building immunity before there's a risk of becoming infected. So if you hatch your own and brood indoors (in the house, basement, garage, inside a coop without access to the outside etc.) you can do it anytime before they go outside, but you have to keep them in for 2 weeks following the shot, to let their immunity build up. If you have broody-hatched chicks that live outside, vaccinate them as soon as you can to give them a good head start. If there's no virus currently in the environment to infect them, they'll still build immunity even though they're outside. I have vaccinated both hand-reared chicks brooded inside, and broody-hatched chicks that lived outside from the start. They are totally fine and I have peace of mind knowing they are protected. There are no risks to the chickens from the vaccine. If you buy chicks instead of hatching, you can buy them pre-vaccinated.
