1. National Institute for Biological Standards and Control, Blanche Lane, South Mimms, Potters Bar, Hertfordshire EN6 3QG, United Kingdom
2. Institut für Infektiologie Krefeld GmbH, D-47805 Krefeld, Germany
3. Institut für Hygiene und Labormedizin, HELIOS Klinikum Krefeld, D-47805 Krefeld, Germany
4. Center for Biologics Evaluation and Research, Food and Drug Administration, Rockville, Maryland 20852-1448