1 Department of Viroscience, Erasmus Medical Center, Wytemaweg 80, 3015 CN Rotterdam, The Netherlands. Electronic address: s.herfst@erasmusmc.nl.
2 University of Veterinary Medicine Hannover Foundation, Research Center for Emerging Infections and Zoonoses (RIZ), Bünteweg 17, 30559 Hannover, Germany. Electronic address: martin.ludlow@tiho-hannover.de.