Immunization against neonatal tetanus in New Guinea. 5. Laboratory assayed potency of tetanus toxoids and relationship to human antitoxin response.

Previous papers in this series have shown that plain toxoids induced early primary antitoxin levels in women in New Guinea that were not significantly different from those induced by adsorbed toxoids but that at the end of 1 year the antitoxin levels differed significantly. Protective levels (not less than 0.01 unit/ml) induced by adsorbed toxoids persisted for more than 3 years. Results of laboratory assays of the toxoids reported in this paper show that per total human immunizing dose, the plain toxoids had 72 or less international units (IU) whereas the adsorbed toxoids had approximately 200 IU. The international "unitage" of these toxoids reflected the persistence of the human protective antitoxin level but not the early primary response. The assay results were in agreement with findings of other workers that the mouse as well as the guinea-pig may be satisfactory for potency assay of adsorbed toxoids. The need for determination of the international unitage of tetanus toxoids used in human studies and the confirmation of relationship of this value to persistence of antitoxin levels is emphasized.