Salmonella Vaccine Study in Oxford (SALVO) trial: protocol for an observer-participant blind randomised placebo-controlled trial of the iNTS-GMMA vaccine within a European cohort by Hanumunthadu, B, Kanji, N, Owino, N, Ferreira Da Silva, C, Robinson, H, White, R, Ferruzzi, P, Nakakana, U, Canals, R, Pollard, AJ, Ramasamy, M