Author contribution: OYu Milushkina, NA Skoblina — research planning and management, material processing, authoring; AB Moiseev — authoring; AA Alsabunchi, SV Markelova, AA Tatarinchik — literature data analysis, material collection and processing, authoring; PO Savchuk, OV Ievleva — material collection.

Compliance with ethical standards: the study was approved by the Ethics Committee of the Pirogov Russian National Research Medical University (Minutes #159 of November 21, 2016), conducted in compliance with the ethical standards provided by the Declaration of Helsinki and the European Community Directives (8/609 EU). Each participant signed a voluntary informed consent form. The participating adults (parents and teachers) were polled voluntary with the help of an online service.