Persistence of Antibodies against Diphtheria, Tetanus, Pertussis, and Poliovirus Types I, II, and III Following Immunization with DTaP Combined with Inactivated Wild-Type Polio Vaccine (DTaP-wIPV).

A follow-up serological study was conducted involving 47 subjects who received 4 doses of diphtheria and tetanus toxoids, combined with the acellular pertussis vaccine (DTaP) together with Salk-type inactivated polio vaccine (DTaP-wIPV), until 6 years of age. All antibody levels declined more rapidly than expected within 3 years after the completion of primary vaccination with the 4th booster dose, and titers persisted until 6 years of age. The positive rate of the IgG antibody against pertussis toxin (PT) was 31.9% (15/47) at 4 years of age, 41.0% (16/39) at 5 years of age, and 40.5% (15/37) at 6 years of age. A significant increase in anti-PT antibodies was observed in 6 subjects, suggesting subclinical infection. Positive rates of antibodies against other targets did not decrease; however, titers of neutralizing antibodies against poliovirus type III decreased in a few subjects. These data suggest the need for an additional preschool booster immunization using DTaP-wIPV.