Determination of anti-HBs IgG of sera from mice primed by the plasmid vectors containing different copies of C3d-P28. BALB/c mice (6 animals per group) were primed im at week 0 with 100 μg of mock DNA (○), pVAON33-S2/S plasmid (●), pVAON33-S2/S-P28 plasmid (△), pVAON33-S2/S-P28.2 plasmid (□), pVAON33-S2/S-P28.3 plasmid (◇) and pVAON33-S2/S-P28.4 plasmid (■), separately. Sera collected at the indicated weeks from each group were diluted 1:10 for determination of specific IgG levels by ELISA. Data were represented as the average for each group of mice. Error bars were represented as 95%CI of the geometric mean for each group of sera.