In the trilogy, we find out that humans are susceptible to a vampire’s control, if the human drinks the vampires blood.
From Protector of the Grey House:
Natalia took deep breaths to try and stop the feelings coursing through her head. It was as if she could hear Mierka’s thoughts in her mind. She breathed deep; her eyes still closed as she pushed away some of the thoughts. The kiss was nice, but Mierka was not her partner. She opened her eyes and looked right at Mierka, who was smiling her secret smile. At the look, everything seemed to coalesce and calm itself into the pretty pattern of the vampire’s smile.
Mierka watched the human’s posture change from one of dominance to one of subservience. Before the blood laced kiss, Natalia had been sitting up straight, and had a look on her face as if she could control the world. Now, she was leaning forward expectantly, looking just at Mierka, as if the vampire knew everything there was to know. Mierka smiled triumphantly.