The Delta variant-specific antibody in plasma was determined as previously described (8 (link)). Briefly, 96-well plates (Thermo Fisher Scientific, Waltham. MA, USA) were coated with 100 ng/well of delta variant spike (S1+S2) or receptor-binding domain (RBD) protein (all from Sino Biological) and incubated overnight at 4°C. The ELISA plates were blocked with 200 µl blocking buffer (0.05% Tween 20 and 10% FBS in PBS) for 1 h at room temperature. Diluted plasma was then added to the plate and incubated for 2 h at room temperature. After incubation, the plates were washed 6 times with 200 µl PBS-T and incubated with anti-monkey IgG and IgM-HRP conjugated antibodies (1:10,000; Rockland Immunochemicals, Gilbertsville, PA, USA) for 1 h at room temperature. After washing, the substrate tetramethylbenzidine was prepared and added to each well. After incubation, the development was stopped with 2N sulphuric acid and the absorbance at 450 nm was read using an ELISA plate reader (BioTek, Winooski, VT, USA).