SUMMARY: Immune response of man to tetanus and diphtheria-tetanus com bined toxoids with different compositions was investigated, with special reference to the long-term immunity. Adsorbed tetanus toxoid with a potency of about 25 IU per human dose endowed a long-lasting immunity sufficient to prevent tetanus to the vaccinees of various age groups, when administered in two doses, followed by a booster injection.Two doses of adsorbed diphtheria-tetanus combined toxoid with potencies of about 20 IU per dose for both components were sufficient to give immunity in infants against diphtheria and tetanus. However, a booster injection should be given within a year to endow a long-lasting immunity.In the case of booster immunization of children with a complete history of im munization against diphtheria and tetanus, a potent long-term immunity was endowed by a single injection of diphtheria-tetanus combined toxoid (plain) with 1 and 3.5 IU per dose, for diphtheria and tetanus components, respectively.

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