Characterization and immunogenicity of a Shigella flexneri 2a O-antigen bioconjugate vaccine candidate by Ravenscroft, N, Braun, M, Schneider, J, Dreyer, A, Wetter, M, Haeuptle, M, Kemmler, S, Steffen, M, Sirena, D, Herwig, S, Carranza, P, Jones, C, Pollard, A, Wacker, M, Kowarik, M