Evolving scientific and regulatory perspectives on cell substrates for vaccine development ; Double Tree Hotel, Rockville, Maryland, USA September 7-10, 1999 ; proceedings of a symposium organized and co-sponsored by the Center for Biologics Evaluation and Research (CBER), the International Association for Biologicals (IABs), the National Institute of Allergy and Infectious Diseases (NIAID), the National Vaccine Program Office (NVPO) and World Health Organization (WHO)