[Diphtheria antitoxin levels in Japanese adults (10-20 years after the last vaccination)].

The serum diphtheria antitoxin levels in Japanese adults were investigated and the persistence of diphtheria toxoid effect was evaluated. The subjects consisted of 56 volunteers (20-31 years of age) who had received regular inoculations of diphtheria and pertussis vaccine (I and II or III phases). They had been immunized according to the Vaccination Law (old version) revised in 1958. The length of time after the last inoculation of diphtheria toxoid was speculated to range from 10 to 20 years. Serum diphtheria antitoxin was determined by passive hemagglutination method. Antitoxin level was 0.025 HAU/ml in 1 subject (1.8%), 0.05 in 2 (3.6%), 0.1 in 2 (3.6%), 0.2 in 8 (14.3%), 0.4 in 7 (12.5%), 0.8 in 12 (21.4%), 1.6 in 7 (12.5%) and more than 1.6 in 17 (30.4%). Results indicated that 55 of 56 (98.2%) possessed a higher level than the prophylaxis standard (0.05 HAU/ml). Since the current Vaccination Law prescribes a lower amount and fewer inoculations than the old law, the number of adults possessing prophylactic antitoxin level may decrease in the future. Further investigation of antitoxin level in adults needs to be continued.