Institute of Social and Preventive Medicine (ISPM), University of Bern, Bern, Switzerland.|Division of Infectious Diseases and Hospital Epidemiology, University Hospital Zürich, University of Zürich, Zürich, Switzerland.|Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States of America.|Division of Infectious Diseases and Hospital Epidemiology, University Hospital Zürich, University of Zürich, Zürich, Switzerland.