National Institute for Biological Standards and Control, Blanche Lane, South Mimms, Herts. EN6 3QG, UK. mforster@nibsc.ac.uk|National Institute for Biological Standards and Control, Blanche Lane, South Mimms, Herts. EN6 3QG, UK. mforster@nibsc.ac.uk|National Institute for Biological Standards and Control, Blanche Lane, South Mimms, Herts. EN6 3QG, UK. mforster@nibsc.ac.uk