[1] Univ Tokyo, Inst Med Sci, Div Virol, Tokyo 1088639, Japan
[2] IMSUT Hosp, Inst Med Sci, Dept Infect Dis & Appl Immunol, Tokyo 1088639, Japan
[3] Univ Tokyo, Inst Med Sci, Adv Clin Res Ctr, Div Infect Dis, Tokyo 1088639, Japan
[4] Natl Ctr Global Hlth, Med Res Inst, Res Ctr Global Viral Dis, Tokyo, Japan
[5] Univ Wisconsin Madison, Influenza Res Inst, Sch Vet Med, Dept Pathobiol Sci, Madison, WI 53715 USA