Center for Applied Molecular Technologies (CTMA), Institut de Recherche Expxc3xa9rimentale et Clinique (IREC), Universitxc3xa9 catholique de Louvain, Clos Chapelle-aux-Champs 30, 1200 Bruxelles, Defence Laboratories Department, Belgian Armed Forces, 1800 Vilvoorde, Belgium, Department of Epidemiology and Hygiene, Military Medical Academy of Sofia, 1000 Sofia, Bulgaria and Epidemiology and Biostatistics Department (EPID), Institut de Recherche Expxc3xa9rimentale et Clinique (IREC), Universitxc3xa9 catholique de Louvain, Clos Chapelle-aux-Champs 30, 1200 Bruxelles, Belgium.