Mice were immunized according to immunization protocol-1 (Table 2). The mice were immunized four times with 100 µl of PBS, 8 ng of hHPV16 VLP or 8 ng of cHPV16 VLP at two-week intervals, in the absence of adjuvant. Ten days after the 3rd and 4th immunization, the mice sera were obtained and analyzed. Panels A and B present the anti-HPV16 L1 IgG titer and neutralization activity after the 3rd immunization, respectively. Panels C and D present the anti-HPV16 L1 IgG titer and neutralization activity after the 4th immunization, respectively. The horizontal bars are median values of the IgG titer and neutralization activity (PBS, n = 6; hHPV16 VLP, n = 15; cHPV16 VLP, n = 15).