An age-stratified serosurvey against purified Salmonella enterica serovar Typhi antigens in the Lao People´s Democratic Republic by Hefele, L, Black, AP, Van Tan, T, Tri Minh, N, Duc Hoang, N, Virachith, S, Muller, CP, Hübschen, JM, Russell, P, Bartholdson Scott, J, Ngoc Minh, CN, Thieu Nga, TV, Baker, S