ZIKV isolate Thailand/1610acTw was kindly provided by the Centers of Disease Control, Taiwan, and propagated in Vero cells (#60013, Bioresource Collection and Research Center, Hsinchu, Taiwan). Virus titers were determined by standard plaque assays. The neutralizing activity of the antibodies against ZIKV was determined. Briefly, two-fold serially diluted antibody solutions (starting from 100 μg/mL or 10 μg/mL) were incubated with 100 PFU of ZIKV for 1 h at 37°C. Antibody-virus complexes were added to Vero cell monolayers in 12-well plates. After 2 h, cells were washed and overlaid with 1.8% (w/v) methylcellulose in DMEM supplemented with 2% FBS and 1% non-essential amino acid. Four days later, plates were fixed with 1% formaldehyde in PBS and stained with the crystal violet for 30 min. Plaques were then counted and compared with PBS control to determine the neutralizing titer. The mouse anti-ZIKV E protein mAb, clone ZV-54 (MABF2046, Merck Ltd.), with known neutralizing activity was also tested (Zhao et al. 2016 (link)).