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It would depend on what they were vaccinated for. If it was for cocci there should be no problems. If it was for Marek's there should be no problems this early on. A chick that is vaccinated for Marek's can contract the virus and not show signs and thus become a carrier. But if the youngest ones were vaccinated as day olds and then brought to her property they are not the carriers of Marek's.
That being said, Marek's probably kills more chickens than any other virus, many times with no symptoms other than death. It can cause internal tumors that you don't see and the birds are so good at hiding illness that by the time you notice anything they are near death. Many times they just die. It is very sad. I have lost 3 from it so far. It can kill up to 80% of your flock, but it usually doesn't happen this soon. More often the birds are closer to 4 months before Marek's will kill them.