Susceptibility to diphtheria in populations vaccinated before and after elimination of indigenous diphtheria in Denmark. A comparative study of antitoxic immunity.

The need for routine diphtheria revaccination is still controversial. Only single cases of diphtheria have been observed in Denmark since the mass vaccinations in the 1940s and the introduction of routine vaccination of infants in 1950, despite the fact that general revaccination has not been recommended. An investigation of antitoxin immunity in 1.188 probands 2-69 years of age, randomly or cluster selected, indicated that immunity in the Danish population is still above the limit generally believed to guard against diphtheria epidemics (around 70% being immune in a population). The highest frequency of susceptibility (46%) was found among females 30-39 years of age. Serum antitoxin concentrations in 529 subjects with documentation for complete primary vaccination 1-40 years previously corresponded to a continuous fall-off. Relative to time from vaccination, individuals born when diphtheria occurred frequently in Denmark had significantly higher antitoxin levels than those born later, and among children susceptibility had increased considerably as compared to observations in the 1950s. These phenomena were explained by the effect of natural immune stimulation previously. The present concept of relatively high frequencies of immune subjects also in other western countries may not be valid in the future, when the number of subjects born after the elimination of indigenous diphtheria increases. General revaccinations seem required to maintain herd immunity.