I misunderstood! I thought you wanted to vaccinate your flock now that there was a Marek's scare. There are a few explinations: 1. It's not Marek's, 2. It is Marek's and the chicks were improperly vaccinated OR did not build resistance (approximately 10% of chicks won't), or, 3. It is a mutated form of Marek's (they exist) which is particularly virulent and the chicks most susceptible to it succumbed. It is very important to note-- according to what I was told by the vet / UoGA, vaccinated birds are harder to test for Marek's and might give a false negative depending on what kind of test is run. It might be worth asking to see if their poultry expert(s) agree with what I was told.