It has been said that "never in the history of human progress has a better and cheaper method of preventing illness been developed than immunization". This is well illustrated by the WHO Expanded Programme on Immunization (EPI) which in developing countries is now preventing nearly a million deaths annually from measles, pertussis and neonatal tetanus, and for which there is a commitment by the WHO and UNICEF to protect all children by immunization by the end of the decade. This enormous undertaking will be facilitated by the rapid advances in molecular biology and recombinant DNA technology,...