Using a human challenge model of infection to measure vaccine efficacy: a randomised, controlled trial comparing the typhoid vaccines M01ZH09 with placebo and Ty21a by Darton, T, Jones, C, Blohmke, C, Waddington, C, Zhou, L, Peters, A, Haworth, K, Sie, R, Green, C, Jeppesen, C, Moore, M, Thompson, B, John, T, Kingsley, R, Yu, L, Voysey, M, Hindle, Z, Lockhart, S, Sztein, M, Dougan, G, Angus, B, Levine, M, Pollard, A