Do you quarantine the chicks to see if they get the disease? At what age? Are my two remaining chickens possibly immune? I am pretty sure my other chickens, being barred rocks from a hatchery, have been vaccinated for different diseases.
You have to quarantine the chicks for at least 3 weeks because the vaccine has to have time to grow a big "army" in a little chick before it goes into the Marek's battle.