Factors influencing immunity against diphtheria in adults.

In about 50% of all adult Germans, serological immunity to diphtheria is lacking even though a vaccination history exists in most cases. In this study, the factors contributing to diphtheria immunity, up to 43 years after the last diphtheria vaccination, were analysed. Serum samples were taken from 287 adults whose complete vaccination history was available, and who had received a standard primary vaccination. The sera were tested by tissue culture neutralization assay for diphtheria antitoxin levels. The time lapse since the last diphtheria vaccination proved to be the most important factor influencing the protection rate. Secondly, the factor last vaccination within the first 3 years of life and, finally, female gender were found to correlate significantly with a low protection rate. No direct effect could be established for age, number of previous vaccinations, occupational contact with patients or travel within any endemic region during the last 10 years. Since diphtheritic infection can occur among previously vaccinated persons, the immunity gap observed among adults should be closed by regular diphtheria boosters. Special attention should be paid to the three groups mentioned. Further investigation is required to determine whether or not a single booster vaccination is sufficient to provide long-term protection for those in the risk groups indicated above.