[Immunity against diphtheria in Taiwan].

The vaccination of combined diphtheria, pertussis, and tetanus (DPT) vaccine provides good immunity against childhood diphtheria in Taiwan. However, a waning protective antibody level has been observed with an increase in age. Therefore, we assessed diphtheria immunoglobulin (DIG) level in Taipei City as representatives of urban dwellers and in King-Shan County as representatives of rural dwellers to evaluate the status of immunity against diphtheria in the population of Taiwan. In total, 1239 serum samples collected from the resident population, age 0-91 years, were detected by toxin neutralization test with VERO cells. The DIG level > or = 0.01 IU/mL was considered to be seropositive and > or = 0.1 IU/mL was considered to be fully protective. The positive rate and fully protective rate for all persons were 79.9% and 36.6%, respectively. The age specific positive rate and fully protective rate for children under 14 years of age were 97.0% and 78.0%, for 20-29 years of age group, were 38.0% and 8.3%, respectively, to be the lowest record among tested age groups. Then, both rates increased proportionately with age. Among those birth cohorts born in the diphtheria immunization era (since 1955), the antibody levels were inversely correlated with age, suggesting a decreased opportunity for exposure to natural diphtheria infection in the recent years and the duration of the vaccine inoculation. This finding indicates the need of a booster vaccination for young adults to ensure a full term protection and a long lasting diphtheria control.