1. Department of Epidemiology and Medicine of Disastrous Events; Medical University; Plovdiv Bulgaria
2. Department of Virology; National Centre of Infectious and Parasitic Diseases; Sofia Bulgaria
3. Viral Hepatitis Prevention Board; Centre for the Evaluation of Vaccination; Vaccine and Infectious Disease Institute; University of Antwerp; Antwerp Belgium
4. Department of Social Medicine and Public Health; Medical University; Plovdiv Bulgaria
5. Department of Epidemiology; National Centre of Infectious and Parasitic Diseases; Sofia Bulgaria