1. Pediatric Clinic, University Hospital “N. I. Pirogov”, 21 “General Eduard I. Totleben” blvd, 1606 Sofia, Bulgaria
2. Department of Healthcare, Faculty of Public Health “Prof. Tsekomir Vodenicharov, MD, DSc”, Medical University of Sofia, Bialo More 8 Str., 1527 Sofia, Bulgaria
3. Medical Faculty, Sofia University St. Kliment Ohridski, 1 Kozyak Str., 1407 Sofia, Bulgaria
4. Clinical Center of Allergology, University Hospital Alexandrovska, Medical University of Sofia, Georgi Sofyiski 1 Str., 1431 Sofia, Bulgaria