1. Heinrich-Pette-Institut, D-20251 Hamburg, Germany and Abteilung Virologie, Universität Heidelberg, D-69120 Heidelberg, Germany
2. University of Yaoundé Teaching Hospital, University of Yaoundé 1, Cameroon
3. Department of Infectious Disease Epidemiology, National Public Health Institute, Helsinki, Finland
4. Heinrich-Pette-Institut, D-20251 Hamburg, Germany and Institut für Virologie, Medizinische Fakultät Carl Gustav Carus, TU Dresden, Germany.