sick

Welcome to BYC. Coughing and gasping can be fairly common in many respiratory diseases. Was there any bloody mucus from the nose and mouth? That is common in ILT which causes gasping and gaping. Infectious bronchitis causes more sneezing and runny noses, but can be worse in young chickens. Aspergillosis can cause silent gasping, and is caused by mold. MG (mycoplasma gallisepticum) and coryza can also cause gasping, and swelling of the face is common. What antibiotic are you using? If it is a virus, it has to run it's course, but antibiotics such as Tylan 50 injectable can help prevent secondary infections. Most sick chickens will not drink enough medication when used in the water because they are too sick. Here is a good link to compare symptoms of the many diseases:
http://edis.ifas.ufl.edu/ps044

 

ILT is a virus, so antibiotics such as Tylan 50 injection or oxytetracycline in the water may only help prevent secondary infections. Most vaccines have to be given several weeks before exposure to become effective. To know for sure what you are seeing, testing of one of the sick ones should be performed, or one should be sacrificed for a necropsy by your state vet if you are in the US. Here is a link about vaccines: http://edis.ifas.ufl.edu/ps030