Pierre Van Damme1, Ann Lindstrand2, Asli Kulane3, Angel Kunchev4. 1. Vaccine & Infectious Disease Institute, University of Antwerp, Belgium. Electronic address: pierre.vandamme@uantwerpen.be. 2. Department of Vaccine and Registries, The Public Health Agency of Sweden, Sweden. Electronic address: ann.lindstrand@folkhalsomyndigheten.se. 3. Department of Public Health Sciences, Karolinska Institute, Sweden. Electronic address: asli.kulane@ki.se. 4. Chief State Health Inspector, Ministry of Health, Sofia, Bulgaria. Electronic address: akunchev@abv.bg.