A multicentre serosurvey on diphtheria immunity in a French population of 1004 subjects.

Diphtheria immunity was determined in serum specimens obtained in 1994 from 1004 subjects seen in emergency departments of three distant French cities. An enzyme immunoassay was used to measure serum diphtheria antitoxin concentrations according to the following criteria: (a) antitoxin < 0.01 IU/ml: susceptibility, (b) 0.01-0.09 IU/ml: basic protection, (c) > or = 0.10 IU/ml: full protection. Among these patients, 20.3% were fully susceptible to diphtheria, 30.3% had basic but doubtful protection and only 49.4% were fully protected. Protection was different by age-groups: 73.5% of the subjects under 40 years of age, 46% between 40 and 65 and 33% over 65 were fully protected. Protection decreased with increasing age (p < 0.001)and was greater for men than women after 40 years of age (p < 0.001). The results of this exploratory study indicate that the enhancement of diphtheria immunity by boosters in adult population should be reconsidered in France as well as in many industrialized countries.