Immunity to diphtheria in children three-to-six year of age in four counties of Taiwan.

An immunization program against diphtheria has been implemented in Taiwan since 1955, using combined diphtheria, pertussis and tetanus (DPT) vaccine. Diphtheria immunoglobulin (DIG) level was assessed in serum samples obtained from 1138 children, aged 3-6 years from north, south, east and central part of Taiwan by the VERO cell neutralization method. Specimens were collected by simple random sampling of residents from Hsinchu, Taichung, Pingtung and Hwalien counties, including both aborigines and non-aborigines. The former lived in one or two villages in each county, and the latter lived in a single village next to the former. Ninety-five percent (1086/1138) had a DIG titre > or = 0.01 IU/ml. There was no significant difference by sex, or by residential area. Seventy-nine percent (901/1138) of the children had completed the primary immunization schedule (at the age of 2, 4, 6 and 18 months), and the prevalence of DIG titre > or = 0.1 IU/ml considered to be long-term protective was as follows: 74.6% for 3-year group; 74.5% for 4-year group; 67.9% for 5-year group; 84.7% for 6-year group (including 52.2% who had had a booster shot at early primary school). These findings show that the diphtheria vaccination program provides good immunity in childhood.