It would be poorly understood. Who would want to vaccinate a sick person anyway? That's flying blind.
How about the medical profession? What you may not realize is that one may be sick but not know he's sick. One may have a small, developing tumor of which he's completely unaware, or be in the beginning stages of an infectious disease that has yet to manifest symptoms. In that case, assailing the immune system with an aggressive vaccine could end up causing dire consequences.