[1] Leicester Royal Infirm, Univ Hosp Leicester NHS Trust, Infect Dis Unit, Leicester LE1 5WW, Leics, England
[2] Univ Leicester, Leicester LE1 7RH, Leics, England
[3] Ctr Dis Control & Prevent, Natl Ctr Infect Dis, Influenza Branch, Atlanta, GA USA
[4] Natl Inst Biol Stand & Controls, Potters Bar EN6 3QG, Herts, England