Chicks hatch Monday, get vaccinated early Tuesday and mailed by noon Tuesday. Most places get them Thursday late morning to early afternoon. I keep biosecurity in mind because even the chicks I am keeping need 3 weeks of isolation from adult birds before they are protected from Marek's.Okay. I tend towards eggs just because it's usually safer because of mareks. But you do have a go point about breeding group size being easier with buying chicks
What is unsafe is to buy older birds that have been outside, vaccinated or not. Any chicken can be infected with and transmit various strains of Marek's. Bringing in a vaccinated, but infected, birds is the worst scenario, as that bird will never show Marek's symptoms, even if you quarantine for a year, but they can pass it along to the rest of the flock, and they could get symptoms and die. Hence the common advice to not mix vaccinated and unvaccinated birds. The advice is sorta-kinda right, sometimes.
Do you vaccinate chicks you hatch?