Measles outbreaks continue to occur in youn unimmunized children and in older individuals who have been receiving measles vaccine. The highest case-fatatlity rates occur in children less than one year old, however current guidelines state that measles vaccine is indicated for infants 12 months and older to avoid the neutralizing effects of passively acquired maternal antibody. Achieving protective immunity across all age groups reqiires several strategies including a better understanding of the optimal age for immunizing young infants, the optimal age for reimmunization, and knowledge about the factors that might enhance or diminish the response to currently available vaccines. Towards this end this laboratory collaborates with several clinical investigators to evaluate the immune response to measles vaccine. In collaboration with Dr. Richard Semba, the immune response to measles immunization with Schwarz vaccine at 6 months of age was compared for infants treated with Vitamin [unreadable] supplementation or placebo at the time of immunization. Surprisingly, the seroconversion rate for the placebo group, 90%, was greater than that seen in the Vitamin A treated group, 85%. Seroconversion rates for antibody negative infants in both groups were identical , 100%. Similarly, studies previously done in collaboration with Dr. Pabst, University of Alberta, indicated that >90% of children born to immunized mothers respond to measles vaccine given at 6 and 12 months. These studies are being extended to assess immunogenicity of additional measles strains and to evaluate the cellular immune response to measles vaccine in this age group. In addition, this laboratory will participate in ACTG protocol 225 to assess the safety and immunogen icity of measles vaccine administered to HIV infected infants at 9 or 12 months of age. Primary vaccine failure and waning immunity also contribute to the spread of indigenous measles. This problem is the focus of several studies in itiated by the CDC; a study of the optimal age for the second dose of measles vaccine (4-5 vs 11-12 years) is currently underway; safety and immunogenicity will be evaluated in 1200 children over the coming year.