Birds vaccinated against Marek's disease will become infected and contagious with Marek's, if they are exposed. The vaccine helps prevent them from developing the tumors that the virus causes, so it improves and hopefully lengthens their lives. Their chicks will NOT get the disease unless it occurs in the flock, so vaccinated but uninfected birds don't spread the virus. In an infected flock, all birds will be infected, and carriers, and only the vaccinated birds will be less likely to die early.
Every infected bird won't die anyway, and 'breeding for resistance' is a strategy for continuing and having chicks in an infected flock. The birds who do live, and are least affected, are bred, and over generations, more may live. ALL still have Marek's disease, and are contagious though.
Coccidiosis 'vaccine' is a different story. It's a low dose of coccidia given to the chicks, hopefully so they develop immunity before being overwhelmed by a large exposure. Nothing to do with Marek's disease.
Mary
Every infected bird won't die anyway, and 'breeding for resistance' is a strategy for continuing and having chicks in an infected flock. The birds who do live, and are least affected, are bred, and over generations, more may live. ALL still have Marek's disease, and are contagious though.
Coccidiosis 'vaccine' is a different story. It's a low dose of coccidia given to the chicks, hopefully so they develop immunity before being overwhelmed by a large exposure. Nothing to do with Marek's disease.
Mary
