[1] Withington Hosp, Manchester Publ Hlth Lab, Meningococcal Reference Unit, Manchester M20 2LR, Lancs, England
[2] Manchester Metropolitan Univ, Fac Sci & Engn, Dept Biol Sci, Manchester M15 6BH, Lancs, England
[3] Publ Hlth Lab Serv, Ctr Communicable Dis Surveillance, Immunisat Div, London NW9 5EQ, England
[4] Natl Inst Biol Stand & Controls, Div Bacteriol, Potters Bar EN6 3QG, Herts, England