Establishment of the first International Standard for human anti-typhoid capsular Vi polysaccharide IgG.

Vi capsular polysaccharide (Vi) conjugate vaccines prevent Typhoid in infants and young children. Several Vi-conjugate vaccines are being developed and an International Standard (IS) for human anti-Vi IgG is required to compare their immunogenicity. The suitability of 16/138 as an IS for human anti-Vi IgG was assessed alongside U.S. reference reagent Vi-IgG_{R1, 2011}, previous candidate IS 10/126 and individual sera. 16/138 is a pool of post-vaccination sera from volunteers receiving either Vi Tetanus Toxoid conjugate vaccine or plain Vi vaccine. Seven laboratories tested the samples in the VaccZyme Human Anti-Salmonella typhi Vi IgG ELISA (Binding Site, n = 7), a biotinylated Vi ELISA (n = 7) and in-house ELISAs (n = 7). Valid estimates were obtained for the potency of all samples when either 16/138 or Vi-IgG_{R1, 2011} were used as a reference in the VaccZyme ELISA. The commutability of 16/138 and Vi-IgG_{R1, 2011} was evident for the VaccZyme ELISA and in-house ELISAs based on a coating of native Vi and a protein. The WHO Expert Committee on Biological Standardization established 16/138 as the first IS for anti-Vi capsular polysaccharide IgG (human) with 100 IU per ampoule and assigned potencies of 163 IU per vial of Vi-IgG_{R1, 2011} and 54 IU per ampoule of 10/126.