DIPHTHERIA ANTITOXIN TITERS 6 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 toroid (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%) indivi duals, who had received 7.5 Lf and in 12 of 87 (14%), who received 15 Lf diptheria toroid (P<0.05). In the original study, the vaccinees wer e enrolled as unimmunized based on their own vaccination histories, bu t many participants had serological evidence of previous immunization. rn the subgroup of 48 truly nonimmune participants, i.e. without prev accination titres and without booster response after the first injecti on, 46% (32% in the 15 Lf group and 58% in the 7.5 Lf group) had antit oxin levels of <0.01 IU ml(-1) 6 years after basic vaccination. Theref ore, individuals who have received basic vaccination with three doses of diphtheria toroid need at least one booster injection 5-10 years la ter. Copyright (C) 1996 Elsevier Science Ltd.