1. Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin—Madison, Madison, Wisconsin, USA
2. Department of Zoology, University of Cambridge, Cambridge, United Kingdom
3. Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo, Japan
4. International Research Center for Infectious Diseases, Institute of Medical Science, University of Tokyo, Tokyo, Japan