1. Faculty of Medicine, Medical University of Varna, Varna, Bulgaria
2. Department of Social Medicine and Organization of Healthcare, Faculty of Public Health, Medical University of Varna, Varna, Bulgaria
3. Department of Hygiene and Epidemiology, Faculty of Public Health, Medical University of Varna, Varna, Bulgaria
4. Department of Health Policy and Management, Faculty of Public Health, Medical University of Sofia, Sofia, Bulgaria