Introducing previously sick chickens back into the flock

[vnork]

Last year my flock went through a bout of respiratory problems. I never had to start antiobiotics and they all seemed to recover but I have always assumed they may now be carriers and any new birds I bring in could catch it. A couple weeks ago I brought home a trio of hens and noticed one was gurgling and wheezing the other two sounded like they were also coming down with it (I never introduced them to my flock). I quarantined them and started injecting them with Tylan 50 1/2cc twice daily. Today is the fifth day of injections and they seem almost 100% back to normal. I am wondering when it would be safe to introduce them into the flock?