Do you mean for Marek's? There is no prevention other than the vaccine. It's carried by wild birds, which you have no control over. Chicks are usually vaccinated right after hatch, but not because there's a magical time. Only because the vaccine is most effective if administered before the body has come into contact with the virus. So, to be safe, people vaccinate them when they are young and before they go outside. However, if there has been no Marek's in the environment where these chickens lived (and where they live now with you), they'll be like babies - "clean", so you can still give them the shot. If they are indeed clean, then they'll develop immunity from the vaccine and will be protected going forward. If they aren't "clean" and have come into contact with the virus at some point, the vaccine won't hurt them at all - it will do nothing. And if the chickens are still alive and healthy, it means that if they did at some point get infected, and did not get sick or die, then they'll be fine anyway. Infection lasts forever and they'll be carriers forever, but not all chickens actually show symptoms. They can be asymptomatic carriers and do just fine. The vaccine won't remove the virus from their systems, it will do nothing. If you do, however, want to get more chickens in the future, it would be best to get them vaccinated before they are exposed to your current chickens, and to keep them indoors for 2 weeks post vaccination to let them develop immunity. If yours are carriers, they can pass the virus on to the new chickens, and while they themselves will be fine, there is no guarantee for the chickens they pass it to. Some percentage of chickens get very sick and even die, and there's no way for you to know how your new chickens will react to the virus. It's hard to know if your current chickens have been infected or not (there are some places that do testing on live birds, but they're hard to find). So, you have two options: 1) do nothing and accept the risk that they may some day get infected and die, or 2) vaccinate them now and give them the possibility of protection. #2 is better safe than sorry, #1 is accepting a risk. Depends on what you're comfortable doing. I don't like taking unnecessary risks myself, so I vaccinate all my animals (and kids). I order the Marek's vaccine from Meyer Hatchery. It's not hard to administer, there are instructions on the label and lots of helpful information, pictures and videos on the internet. Unfortunately there's no convenient small quantity for backyard chickens - only the large quantities for commercial use, but that's just how it is.
