1. *Department of Pathology, National Institute of Infectious Diseases, Musashimurayama, Tokyo, Japan;
2. †Department of Vaccine Immunology, Hokkaido University Graduate School of Medicine, Sapporo, Japan;
3. ‡Nebuta Research Institute for Life Sciences, Aomori University, Aomori, Japan; and
4. §Influenza Virus Research Center, National Institute of Infectious Diseases, Musashimurayama, Tokyo, Japan