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

Abstract

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 booster doses of diphtheria toxoid may not achieve an adequate immune response.