Figure 3. Neutralizing antibody levels to diphtheria toxoid (DT) of mice immunized with the diphtheria-tetanus-pertussis (DTP) vaccine. The lines show the average antibody titers detected at (A) 14 days after each vaccine injection during the primary immunization schedule and (B) before (pre-booster) and 14 days after boosting at 2, 4, and 6 months (mo) after the primary immunization schedule. The dashed line in A and B represents the protective neutralizing antibody level (>0.1 IU/mL). Points represent each individual antibody level, which was tested at least twice for each sample. *P < 0.05 compared to 2 doses. ^#P < 0.05 compared to 3 doses. ⁺P < 0.05 compared to pre-booster. The paired t-test was used for statistical analysis.