Immunity to Diphtheria in an Urban Population

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Abstract
Immunity against diphtheria is poorly understood since protection is the result of a complex interaction between host and environment, and technical problems with the present antitoxin assays exist. A radioimmunoassay which permits a reproducible estimation of antitoxin concentration and relative avidity was employed to characterize the immunological response to diphtheria of 195 healthy urban children. The antitoxin concentration in cord serum was equal to or greater than the concentration in maternal serum and decreased rapidly during the first two months of life. The first diphtheria-tetanus-pertussis inoculation was not immunogenic enough to initiate significant antitoxin production in infants without maternal antitoxin or in older children. A significant increase in antibody concentration and affinity was found after the second, third, and fourth toxoid administrations. More than four injections did not alter concentration, affinity, or duration of active immunity. Correlation with recent epidemiologic studies suggested that a serum contained a "protective" concentration of antitoxin if it bound more than 100 ng of diphtheria toxin nitrogen per milliliter of serum (approximately 0.1 IU of antitoxin). By this criterion, among the study group only 74% of the healthy children who had received three immunizations and 84% of the children who had received four or more inoculations were "protected." Antitoxin concentrations observed in serum from children with chronic diseases who had received appropriate immunizations were lower than in serum from appropriately immunized healthy children. These observations suggest that the immunologic status of urban children against this disease is not optimal and should serve as a stimulus to improve both patient education and immunization practices.