All of your day old chickens should be vaccinated against a number of diseases such as Marek’s disease. The antibodies that are produced as a result can help the chickens fight the disease if and when they become exposed to it.
Passive immunity can be passed on from the hen to the egg. If the hen had a higher level of antibodies they can be easily passed to the egg yolk and the fluids in the egg. This helps the chick to become resistant towards many diseases for a few weeks. If the little chickens have high levels of antibodies following their birth, then giving them a vaccination may, in fact, result in a reduced state of response.