An appropriate vaccine is essential for a global pandemic preparedness and response. Hemagglutination inhibition assays reveal that the 2009 pandemic H1N1 viruses are antigenically homogeneous and distinct from currently circulating seasonal H1N1 viruses. Based on antigenic and genetic analysis and epidemiologic information, the WHO selected the A/California/7/2009 (H1N1) virus as the reference virus for vaccine development. We will evaluate the immunogenicity and infectivity of the live attenuated reassortant H1N1 virus containing the surface glycoproteins from the California/7/2009 (H1N1) virus and internal protein genes from the A/Ann Arbor/6/60 cold-adapted (ca) virus in clinical trials.