[1] Univ Leicester, Leicester, Leics, England
[2] Natl Inst Biol Stand & Controls, Potters Bar EN6 3QG, Herts, England
[3] Hlth Protect Agcy, Colindale, England
[4] Ctr Dis Control & Prevent, Atlanta, GA USA
[5] US FDA, Rockville, MD 20857 USA
[6] Natl Inst Control Pharmaceut & Biol Prod, Beijing, Peoples R China