Assessing the impact of a Vi-polysaccharide conjugate vaccine in preventing typhoid infection among Bangladeshi children: A protocol for a phase IIIb trial by Theiss-Nyland, K, Qadri, F, Colin-Jones, R, Zaman, K, Khanam, F, Liu, X, Voysey, M, Khan, A, Hasan, N, Ashher, F, Farooq, Y, Pollard, A, Clemens, J