Animal and human studies have shown that the prime-boost immunization strategy using malaria antigens expressed in plasmid or viral vectors induces strong cellular immune responses. An immunization regimen with the malaria vaccines DNA ME-TRAP followed by MVA ME-TRAP induced strong T cell responses in adults in the United Kingdom (UK) and in the Gambia but did not provide significant clinical protection against infection. The investigators assessed two new vaccines which utilize a similar immunization strategy but a different malaria antigen, a circumsporozoite (CS) protein. The entire CS protein was expressed either in a modified vaccinia virus Ankara (MVA) CS, or an attenuated fowlpox virus strain (FP9) CS.