Leo G Visser1, Olivia Veit2,3,4, Lin H Chen5,6. 1. Department of Infectious Diseases, Leiden University Medical Center, Leiden, The Netherlands. 2. Department of Medicine, Swiss Tropical and Public Health Institute, Basel, Switzerland. 3. University of Basel, Basel, Switzerland. 4. Institute of Epidemiology, Biostatistics and Prevention, University of Zürich, Switzerland. 5. Division of Infectious Diseases and Travel Medicine, Mount Auburn Hospital, Cambridge, MA, USA. 6. Faculty of Medicine, Harvard Medical School, Boston, MA, USA.