Titers of anti-Nic IgG, IgG1, IgG2a, IgG2b, and IgG3 and the Th1/Th1 index. Eight female BALB/c mice in each group were injected on days 0 (primary injection), 14 (first booster), and 28 (second booster) with either the PLGA vaccine or the PLA vaccine as shown in Table 1. Titers of anti-Nic IgG were measured for blood collected from each immunized group on days 13, 27, 42, and 118. Titers of subtype antibodies, including IgG1, IgG2a, IgG2b, and IgG3, were measured for blood collected from each group on day 118. The Th1/Th2 index, which represented the polarization of the immune response, was calculated according to the titers of subtype IgGs. ** indicates that the p-value was less than 0.05, and *** indicates that the p-value was less than 0.01.