Chick immunizations

[troutfarm]

After losing my three white rock hens this fall/winter but all the rest of my birds are okay, I was told by a vet that perhaps those 3 (who were all from the same brood) perhaps did not receive immunizations they should have at the hatchery. Before I get more chicks this spring I want to make sure they will have the immunizations they need. Is there any way to insure this? What immunizations should they have?

Thanks for any help!