Inserm UMR 1181 Biostatistics, Biomathematics, Pharmacoepidemiology and Infectious Diseases (B2PHI), F-75015, Paris, France, Institut Pasteur, UMR 1181, B2PHI, F-75015, Paris, France, Univ. Versailles St Quentin, UMR 1181, B2PHI, F-78180 Montigny-le-Bretonneux.