Immunity to diphtheria among adults older than 65 years

BACKGROUND AND OBJECTIVE: To know the immune state against diphtheria through a survey, a consultation of the vaccination documents and a seroepidemiologic study of an old population. PATIENTS AND METHOD: The immunization status and the prevalence of antidiphtheria antibodies were determined in a representative sample (n = 231) of this population group (> 65 years). Concentrations of toxin antibodies were measured using a commercial ELISA kit. Samples with titres > 0.1 Ul/ml were considered protective. RESULTS: In the vaccination documents we only found 19 old individuals vaccinated against tetanus-diphtheria. 47.1% had no protective titres. Geometric mean titres were 0.12 Ul/ml. There was no statistically significant difference between the variables age and sex and the prevalence of antibodies. 84.4% of the 122 seropositives; did not report having received diphtheria vaccine and neither had documents about the vaccination. CONCLUSION: There were almost no documents about diphtheria vaccination. Therefore, the serologic study is the only reliable way to know the immunity status against diphtheria in elderly people. Approximately 50% of this population need primary immunization. The rest of this group need to have completed antidiphtheria vaccination or a dose booster within a period of 2 years in order to guarantee protective antibody levels against diphtheria.