WHO/BS/2014.2234 Collaborative study to evaluate a candidate IS for anti-Typhoid capsular Vi polysaccharide IgG (human)

Overview

Candidate International Standard (IS) 10/126 is a freeze dried preparation of 0.5 mL pooled sera, taken from 9 volunteers immunised with a Vi polysaccharide (Vi) tetanus toxoid conjugate vaccine and contains IgG directed against Vi and lipopolysaccharide of Salmonella enterica subspecies enterica serovar Typhi. A collaborative study comprising ten laboratories from seven countries assessed the suitability of candidate IS 10/126 for Vi ELISAs, alongside reference reagent for human anti-Vi IgG, Vi-IgGR1, 2011 provided by the Center for Biologics Evaluation & Research (CBER; Szu et al., Vaccine 2013;31:1970–1974). Both reference materials were tested in a commercially available Vi ELISA kit (n=3), an ELISA based on CBER reference reagent Vi Lot no. 05 (Vi 05 ELISA; n=7) and in-house ELISAs based on locally prepared Vi (n=4). Of the latter two ELISAs, 9 out of 11 assays were based on slight modifications of the procedure by Szu et al. (2013). Candidate IS 10/126 is suitable for use in Vi ELISAs. The relative potency (RP) of candidate IS 10/126 compared to Vi-IgGR1, 2011 depended on the ELISA format. The GM of the RP based on the kit ELISA was calculated as 0.65 (geometric coefficients of variation (GCV) 15%; n=3), for the Vi 05 ELISA as 1.40 (GCV 67%; n=6) and for the in-house ELISAs as 0.41 (GCV 77%; n=4). The overall geometric mean (GM) of the RP was calculated as 0.80 (95% Confidence Interval 0.52 - 1.24; GCV 105%; n=13). Based on the RP of all ELISA data, we recommend that 10/126 be established as the 1st IS for anti-Typhoid capsular Vi polysaccharide IgG (human), with an assigned potency of 0.5 IU per ampoule or 1 IU mL^-1