I placed 6 eggs under my broodie silkie hen 21 days ago. Yesterday morning, we awoke to 4 beautiful and healthy-looking baby chicks. The 2 remaining eggs have not hatched and I don't see and progress yet. I am wondering if I should wait to give the chicks the mereks vaccine for a few more days, to see if the other two eggs hatch? I only have one vial of vaccine so I would prefer not to have to buy another if not needed. Thank you!