For a backyard flock, I would not vaccinate for anything at this point. When I get hatchery chicks, I get the Mareks vaccines for them. They are best given as day olds before they could possibly be exposed. Do you have neighbors with chickens or have any large poultry operations nearby? When you get chickens from another person, they can come from farms where they might have been exposed to something, such as a chronic respiratory disease. Look for watery, bubbly eye, nasal drainage, or sneeze/cough. Most people quarantine new chickens for a month, to look for any symptoms of illness or disease, to check for lice or mites, and to get a fecal float done by a vet to look for worms or coccidiosis. If you see any illness, isolate the chicken, and if a death occurs, send the body to your state vet for a necropsy and testing. Good luck with your flock.