I talked to the man selling Mareks vac. and he said you need to vac, day old chicks as it becomes less effective after each additional day of age, he also said the med. feeds are not very useful as they are heated to such a temp as to make the vac. not to effective. He also said they can not get infected by other birds that have been treated because of the type of vaccine they use. You also should not cut the tablets in half because you can't be sure of the amount of med. you are getting per slice of tablet. Any feed back from others? Momma has been hiding her babes and I think they are more then a day old so it is to late to vaccinate, right?