Booster effect of low doses of tetanus toroid in elderly vaccinees

In order to improve the immunity to diphtheria, the recommended booster dose of diphtheria/tetanus vaccine for adults in Sweden was changed in 1986 from 0.5 mi of tetanus vaccine with a small diphtheria dose to 0.25 mi of a diphtheria/tetanus vaccine containing 7.5 Lf tetanus toroid and 30 Lf diphtheria toxoid/ml. This change resulted in an increase in the dose of diphtheria toroid from 0.5 Lf to 7.5 Lf, but a decrease in the recommended booster dose of tetanus toroid from 3.75 Lf to 1.9 Lf. The aim of the present study was to investigate whether this lower dose of tetanus toroid was also sufficiently protective for elderly people. Two hundred adults (median age 76 years, range 60-92 years) with no history of tetanus vaccination during the past 10 years volunteered for the study. One hundred two vaccinees were inoculated with 1.9 Lf tetanus toroid (0.25 ml) and 98 with 3.75 Lf tetanus toroid (0.5 mi). Paired serum samples were analysed by the toxin-binding inhibition assay. Side effects were few and mild, without significant differences between the groups. Response rates were similar, with the 3.75 Lf dose eliciting a marginally higher antitoxin response. The prevaccination geometric mean titre was the same for both groups: 0.03 IU/ml. Postvaccination geometric mean titres were 1.18 IU/ml for the 3.75 Lf group and 1.93 IU/ml for the 7.5 Lf group, respectively (difference not significant). Forty-seven percent of the vaccinees had a prevaccination titre of less than or equal to 0.01 IU/ml. Postvaccination, 85% had a titre >0.01 IU/ml. Booster vaccination with tetanus vaccine containing only 1.9 Lf of tetanus toroid was thus found to induce an excellent immune response in elderly people, with few side effects resulting.