Role of highly avid antitoxic antibodies in the estimation of insusceptibility to diphtheria infection

A procedure was developed to determine the avidity of antitoxic antidiphtheria antibodies, by using the enzyme immunoassay. It was shown that both total antibodies and their avidity might be estimated in one test. A study of sera from healthy vaccinees could define differences in the time course of changes in the accumulation and loss of both total and highly avid antitoxic antidiphtheria antibodies. Maturation of highly avid antibodies took place more slowly than the total number of antibodies increased whereas loss of highly avid antibodies occurred much more quickly than the total count of antibodies decreased. Five years following regular revaccination, the antibody avidity index achieved its minimal (critical) level while the content of antitoxic antibodies was reduced to critical values (0.01 IU/ml) at 7-8 years after revaccination. Risk groups were identified among the examinees. These included subjects aged over 50 years, medical workers, industrial workers, particularly those engaged in hazardous occupations, subjects with chronic somatic and infectious diseases, and those vaccinated against diphtheria more than 5 years ago.