[1] Univ Hosp Leicester, Infect Dis Unit, Leicester Royal Infirm, Leicester, Leics, England
[2] WHO, Collaborating Ctr Influenza Reference & Res Melbo, Melbourne, Vic, Australia
[3] Univ Melbourne, Dept Microbiol & Immunol, Melbourne, Vic 3052, Australia
[4] WHO, Inst Vaccine Res, CH-1211 Geneva, Switzerland