Diphtheria antitoxin titres six years after basic immunization of adults.
Diphtheria antitoxin titres were analysed in 160 adults (median age 59 years, range 34-70), who completed basic vaccination with three doses of 7.5 Lf or 15 Lf of diphtheria toxoid (D) in a previous vaccination trial in 1987, in serum samples drawn 6 years later. The median titre had decreased from 3.2 IU ml-1 in the post vaccination samples to 0.2 IU ml-1 after 6 years in the 15 Lf group and 0.1 IU ml-1 in the 7.5 Lf group. An antitoxin titre of < 0.01 IU ml-1, a level usually considered to give no safe protection, was found in 21/73 (29%) individuals, who had received 7.5 Lf and in 12 of 87 (14%), who received 15 Lf diphtheria toxoid (P < 0.05). In the original study, the vaccinees were enrolled as unimmunized based on their own vaccination histories, but many participants had serological evidence of previous immunization. In the subgroup of 48 truly non-immune participants, i.e. without prevaccination titres and without booster response after the first injection, 46% (32% in the 15 Lf group and 58% in the 7.5 Lf group) had antitoxin levels of < 0.01 IU ml-1 6 years after basic vaccination. Therefore, individuals who have received basic vaccination with three doses of diphtheria toxoid need at least one booster injection 5-10 years later.