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Many malaria vaccines are currently in development, although very few have been evaluated for efficacy in the field. Plasmodium falciparum multiple epitope (ME)-thrombospondin-related adhesion protein (TRAP) candidate vaccines are designed to potently induce effector T cells and so are a departure from earlier malaria vaccines evaluated in the field in terms of their mechanism of action. ME-TRAP vaccines encode a polyepitope string and the TRAP sporozoite antigen. Two vaccine vectors encoding ME-TRAP, plasmid DNA and modified vaccinia virus Ankara (MVA), when used sequentially in a prime-boost immunisation regime, induce high frequencies of effector T cells and partial protection, manifest as delay in time to parasitaemia, in a clinical challenge model. Methods and Findings A total of 372 Gambian men aged 15-45 y were randomised to receive either DNA ME-TRAP followed by MVA ME-TRAP or rabies vaccine (control). Of these men, 296 received three doses of vaccine timed to coincide with the beginning of the transmission season (141 in the DNA/MVA group and 155 in the rabies group) and were followed up. Volunteers were given sulphadoxine/pyrimethamine 2 wk before the final vaccination. Blood smears were collected weekly for 11 wk and whenever a volunteer developed symptoms compatible with malaria during the transmission season. The primary endpoint was time to first infection with asexual P. falciparum. Analysis was per protocol. DNA ME-TRAP and MVA ME-TRAP were safe and well-tolerated. Effector T cell responses to a non-vaccine strain of TRAP were 50-fold higher postvaccination in the malaria vaccine group than in the rabies vaccine group. Vaccine efficacy, adjusted for confounding factors, was 10.3% (95% confidence interval, 22% to + 34%; p = 0.49). Incidence of malaria infection decreased with increasing age and was associated with ethnicity. [ABSTRACT FROM AUTHOR]

Successful vaccination against intracellular pathogens, including liver-stage Plasmodium falciparum, will require induction of strong antigen-specific T lymphocyte responses. The multiple epitope (ME)-thrombospondin-related adhesion protein (TRAP) construct includes CD8+ and CD4+ T cell epitopes from pre-erythrocytic P. falciparum antigens fused in-frame to the entire pre-erythrocytic antigen TRAP. Three carriers for this construct--plasmid DNA and 2 recombinant nonreplicating poxviruses (modified vaccinia virus Ankara [MVA] and fowlpox strain 9 [FP9])--were administered at 3-week intervals in a heterologous prime-boost combination to 29 Gambian men aged 18-45 years. Doses of DNA ME-TRAP, MVA ME-TRAP, and FP9 ME-TRAP were 2 mg and 1.5 × 108 and 1 × 108 plaque-forming units, respectively. DNA ME-TRAP was injected intramuscularly; MVA ME-TRAP and FP9 ME-TRAP were injected intradermally. There were no clinically relevant laboratory abnormalities and no severe or serious adverse events related to vaccination. DNA/MVA and FP9/MVA regimens were the most potent inducers of circulating effector T cells seen to date in sub-Saharan Africa. Twelve months after the final vaccination, a single booster vaccination expanded the effector T cell pool to a similar or higher magnitude than that after the primary vaccinations. These results highlight optimized combination regimens with general relevance to the development of vaccines targeting intracellular pathogens. [ABSTRACT FROM AUTHOR]