1 Department of Pathobiological Sciences, Influenza Research Institute, School of Veterinary Medicine, University of Wisconsin-Madison, 575 Science Drive, Madison, WI 53711, USA.
2 Department of Pathobiological Sciences, Influenza Research Institute, School of Veterinary Medicine, University of Wisconsin-Madison, 575 Science Drive, Madison, WI 53711, USA; Division of Virology, Department of Microbiology and Immunology, International Research Center for Infectious Diseases, The Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan; Research Center for Global Viral Diseases, National Center for Global Health and Medicine, Tokyo 162-8655, Japan; The University of Tokyo, Pandemic Preparedness, Infection and Advanced Research (UTOPIA) Center, Tokyo 108-8639, Japan. Electronic address: yoshihiro.kawaoka@wisc.edu.