It seems to help most dramatically after the bug has set in. I often have allergy and drainage year-round, so I don't usually recognize anything more serious until it's too late. :-(
Boneset also supposedly stimulates the immune system in some way, so maybe it could prevent or damper whatever you may have "hiding out" in your system. But, because of some liver toxicity concerns, it's definately not a daily, health-maintenance kind of brew. Just the occasional, acute symptoms remedy.