We have an established flock of mixed breed pullets and 1 cockerel that are 6 months old. We got them as day old chicks from a hatchery and had them vaccinated for Mareks. We are in the process of preparing for my son's 4-H project. We will be getting 4 4-month old pullets from a reputable breeder. They have not been vaccinated. I can't even imagine how devastated my son would be if the birds became ill and we hadn't done everything possible to protect them. I would appreciate any advice from experienced chicken keepers out there about whether we should vaccinate (if you even can) the new birds coming in. If it makes a difference, the new birds are coming from a town 2 hours away. Thank you very much for any advice.