I have some birds that are pushing a year old now and 7 chicks in the brooder and 7 more on order. As the near year olds were my first birds I didn't know much about marek's and prevention and all that when I got them. I want to make sure the whole flock is protected. The 7 chicks on order will receive the vaccination from the hatchery. The ones in the brooder I am checking with the people I got them from to see if they were vaccinated or not. They are approx 2 weeks old now. I should still vaccinate them right if they are not and what about the older ones? I have read where they develop natural immunities to it after 5-6 months of age and its no longer a worry in those birds. Is that the case or should I hit them with it too?