Immune Status and Booster Effects of Low Doses of Diphtheria Toxoid in Swedish Medical Personnel

During a diphtheria outbreak among Swedish alcoholics in 1984-1985, only 60% of 328 medical staff at risk for exposure had diphtheria antitoxin titers ≥0.01 IU/ml, which is usually considered to give relative protection. 21% had levels between 0.01-0.09 IU/ml and the remaining 39% had titers ≥0.1 IU/ml. Booster doses of 0.5 Lf-12.5 Lf of diphtheria toxoid were given to 450 vaccinees. Of 72 individuals with low pre-immunization titers, who were immunized with ≤2.5 Lf diphtheria toxoid, 40% failed lo reach ≥0.1 IU/ml when analyzed 4 weeks after vaccination. Local tenderness and swelling >5 cm at the site of injection or general discomfort was found in 11% of those immunized with low dose diphtheria toxoid (≤2.5 Lf). When the dose 0.5 Lf of diphtheria toxoid was combined with tetanus toxoid (3.75 Lf) the frequency of adverse reactions increased to 34% (p<0.001). The study shows that vaccination status in medical personnel must also be continuously examined in countries where diphtheria is rare and that low b...