Since the development in 1961 by Hayflick and Moorhead of a human diploid cell strain (HDCS) culture system for isolating viruses, HDCS-derived human vaccines have been licensed worldwide for polio IPV and OPV (multiple strains), rabies (Pitman-Moore L503 3M strain); rubella (RA27/3 strain); measles (Edmonston-Zagreb strain); varicella-zoster (Oka strain); mumps (Rubini strain) and hepatitis A (HM-175, CR-326-F', and GBM strains). Many of these widely-used vaccines now have at least a 20-year record of safety after extensive, ongoing pharmacovigilance. Serious vaccine-associated events are rare, and appear to reflect the activity of the live viruses replicated in the HDCS or of the inactivated viruses or other proteins added during manufacture. There is no credible association of reactions to the HDCS substrate or a hypothetical contaminant derived from it.