The effect of a single booster injection of an adult formulation of a combined
diphtheria-tetanus vaccine (Td) on
diphtheria-specific immunity was evaluated. The booster injection, containing 2 IU
diphtheria toxoid per dose, was given as part of the
surgical wound management for adults with open
soft tissue injuries.
Diphtheria antitoxin concentrations were determined in serum samples from 534 patients (199 women and 335 men, aged 18-70 years) using an
enzyme immunoassay and a tissue culture toxin neutralization assay. Seroimmunity against
diphtheria toxin was classified at three levels: susceptibility, basic protection, and full protection against the toxic manifestations of the disease. Before vaccination, 27.1% of the subjects were susceptible to
diphtheria, 26.5% had basic protection, and 46.4% were fully protected. Six weeks (minimum 25 days, maximum 98 days) after a single booster injection, 89.7% of the subjects achieved full protection against
diphtheria, and only 3.9% had
antitoxin levels below the protective level. The median increase from the prevaccination to postvaccination
antitoxin concentration was found to be 14-fold (4.4-47; quartiles Q25 to Q75). The change in
antitoxin levels after revaccination was higher in older age groups (P< 0.001), whereas neither sex (P = 0.86) nor the country of previous immunization with a different national immunization schedule (P=0.61) had a significant influence on the revaccination effect. Systemic adverse reactions were rare, and local reactions of clinical significance were reported in only 1.9% of subjects.