Vibriocidal assays were performed as described previously15 (link)22 (link). Serially diluted heat inactivated test sera were incubated with target strains O1 Inaba (T19749) and Ogawa (X25049) and guinea pig complement (Sigma Aldrich, catalog # S1639). Titers were defined as the reciprocal of serum dilution that resulted in 50% or greater reduction in O.D. compared to serum free controls15 (link)22 (link). An internally generated, pooled serum sample served as the positive control and helped in accounting for inter-experimental variation. Samples were tested in duplicates. The threshold for inter- and intra-experimental variation was set at 2-fold. Seroconversion was defined as ≥4-fold rise in vibriocidal titers compared to baseline, following convention. Given the high inter-laboratory variation in vibriocidal titers, we standardized our vibriocidal protocol by comparison with de-identified serum samples with known vibriocidal titers obtained by a previous publication from Haiti15 (link).
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