Concentrations of dengue nAb in Vero cells were measured by PRNT as published [34 (link)]. The CV-1-Fc cell PRNT assays were performed similarly. In brief, identical concentrations of virus for each serotype were used in parallel CV-1-Fc and Vero PRNTs and mixed 1:2 with 2-fold serial dilutions of sera for a final concentration range of 1:10 to 1:20480. Duplicate wells of each sample were evaluated when sufficient sera were available. After 6 days, cell monolayers were fixed and incubated with DENV serotype-specific mouse antienvelope mAbs from Biotem (France): anti-DENV1 (D2-1F1-3), anti-DENV2 (3H5-1-12), anti-DENV3 (8A1-2F12), and anti-DENV4 (1H10-6-7), followed by incubation with alkaline phosphatase-conjugated goat antimouse IgG (Jackson ImmunoResearch). Plaques were visualized with SIGMAFAST BCIP/NBT alkaline phosphatase substrate in levamisole solution. Dengue virus plaques in CV-1-Fc cells for each serotype were typically smaller than plaques from parallel Vero assays but had the shape and density consistent with Vero DENV plaques (data not shown). The number of plaques was counted in high-resolution images of each well.