I recently bought 10 chicks from TSC. Of course, coming from TSC they are not vaccinated. They are around 3 weeks old now. I just ordered 60 chicks from a hatchery and they will be arriving in the next week. I got the hatchery chicks vaccinated for Mareks but I am worried about the older TSC chicks getting the younger ones sick because they have not been vaccinated for Mareks. Is it too late to vaccinate 3 week old chicks for Mareks?