Long-term persistence of anti-diphtheria toxin antibodies among adults in Israel. Implications for vaccine policy.

Vaccination against diphtheria has essentially led to the disappearance of the disease in Israel. However, in other countries with high immunization coverage, isolated cases and small outbreaks have occurred in adults. Immunity following vaccination or natural exposure to toxigenic strains of C. diphtheriae is conferred by serum antibodies to diphtheria toxin. Since booster doses of diphtheria toxoid are recommended every ten years in adults, this raises the question of persistence of protective levels of anti-diphtheria toxin antibodies. In this study we assessed a possible age-related decline in anti-diphtheria toxin antibodies among adults in Israel. The study population comprised random samples in three age groups: 263 male recruits aged 18-19 years, 116 male reserve soldiers aged 25-35 years and 153 aged 41-51 years. Anti-diphtheria toxin antibody levels were measured by means of ELISA. Results indicate that 64.3% (95% CI = 58.5-70.1%) of those aged 18-19 had anti-diphtheria toxin levels in excess of 0.1 IU ml-1, whereas the corresponding figures for ages 25-35 and 41-51 were 32.8% (95% CI = 24.2-41.3%) and 15% (95% CI = 9.4-20.7%). However, even in the oldest age group, 95.4% (95% CI = 90.8-98.1%) had antibodies above the presumed protective level of 0.01 IU ml-1. Although these results indicate a significant age-related decline in anti-diphtheria toxin antibodies in vaccinated subjects, most had apparently protective levels. The absence of cases suggests that vaccine-induced immunity is long-lasting. However the immune status of the population should be carefully monitored.