"Undetectable" can mean one of two things. Either the virus has gone completely, in which case it will not come back (about 99% sure), or it is there in too small a quantity to be detected, in which case it will start replicating like mad once you come off tx and you will soon have thousands of copies. By soon, I mean within a month or so. This is why you have a test 6 months after you finish to see which kind of undetectable you were.
Hi - perhaps this is belaboring the obvious, but I wanted to add that following this forum can be very helpful with managing your depression, in case it does get worse during tx.
Treatment may increase your depression. You should be prepared for this eventuality by discussing it with your doctor.
If you become a sustained viral responder (SVR) you are "cured." You will always carry the hep c antibody but there will be no live virus in your system. Mike