The Vero proteins in samples were measured by ELISA. The anti-Vero guinea pig IgG (3 μg/mL into NaHCO3, pH 9.6) was coated with 100 μL/well into a 96-well microplate (Corning, USA) overnight at room temperature. After three washes with PBST (PBS pH 7.2, 0.05% Tween 20), the plate was blocked with blocking buffer (3% BSA in PBST) for 1 h. Host cell protein standard and samples were serially diluted with blocking buffer and added into wells with 100 μL/well after three washes and incubated for 1 h at 37 °C. The plate was washed for three times and anti-Vero rabbit IgG (1:1000 in blocking buffer) was added with 100 μL/well and incubated for 1 h at 37 °C. After three washes, HRP-conjugated goat anti-rabbit serum was added and incubated for 1 h at 37 °C. After washes with PBST, TMB solution was added with 100 μL/well and incubated for an appropriate time at 37 °C before 1 M H2SO4 solution was added to stop the reaction. Color development was measured at 490 nm by a Thermo microplate reader (Thermo, USA).