The problem of Marek's vaccination is primarily one of cost and feasibility. The vaccine is available to anyone who wishes to purchase it but it comes in such huge amounts of dosages that unless you run a large poultry operation, those doses go to waste since they need to be used up as soon as the vaccine vial is opened, and it only comes in 1000 dose vials. It has to be given a few weeks before any chickens are introduced into a flock where Marek's is present.
For this reason, small flock owners have their new chick orders vaccinated at the hatchery just before they are shipped. The chicks need a few weeks to develop the antibodies before you expose them to the flock with Marek's.
Chicks have a small window during the first week or two after hatching where they are most receptive to developing antibodies, you can vaccinate at any age, but it does no good if the chickens have already been exposed.
Introducing new chicks to a flock that carries Marek's will expose them to the virus. While there isn't any evidence they can develop resistance by exposing them during the first two weeks after hatch, there is a possibility of this. After this window passes, older chicks will be infected with the virus and carry it from then on. Resistance isn't the same as immunity.
Chickens carrying these avian viruses often live mostly normal lives until tumors at a later date sickens and kills them. Many flocks carry these viruses and the owners are never aware of it because they slaughter their chickens for food before symptoms ever occur.
Your chicks are a couple months old and it's not practical to vaccinate them. You can put them in with the flock any time and hope by good management and hygiene, they will develop resistance and live out normal lives. I have an avian virus in my flock, and some of my chickens are ten to twelve years old and still going strong.
