Increased dosage of diphtheria toxoid for basic immunization of adults

Basic immunization of adults with increased dosages of a diphtheria toxoid vaccine (2100 flocculation units (Lf)/mg) was evaluated. Three injections of 7.5 Lf or 15 Lf diphtheria toxoid were given to 243 adults who had a history of no more than one previous vaccine injection. Systemic reactions were rare in both groups. Following the first two injections, local reactions (greater than 5 cm) were observed in 6-14% of the adults. After the third injection, 35% of adults in the 15 Lf group reported a local reaction (greater than 5 cm) compared to 11% in the 7.5 Lf group (p less than 0.001). The 15 Lf dose elicited a better antitoxin response than the 7.5 Lf dose. In a seronegative subgroup including 65 vaccinees who showed no booster response to the first vaccination, 79% had a postvaccination titer of greater than or equal to 0.1 IU/ml and 28% a titer of greater than or equal to IU/ml after the third injection of 7.5 Lf. The corresponding numbers in the 15 Lf group were 94% and 44%, respectively. The study demonstrates that 7.5 Lf and 15 Lf diphtheria toxoid of high purity can safely be given to adults for basic immunization. The higher dose is more immunogenic but local reactions increase after the third injection.