ELISA was used to determine the anti-HBoV antibodies, as described elsewhere [40] . The purified HBoV VLPs were used as coating antigen (0.125 mg/mL). The absorbance of each serum sample was read at 450 nm and the mean values of the duplicate samples were calculated. Sera pooled from ten samples showing HBoV-specific IgG responses were used as the internal references in all experiments.