Prevalence of diphtheria and tetanus antibodies among adults in Singapore: a national serological study to identify most susceptible population groups.

BACKGROUND: In view of waning antitoxin titres over time after the last vaccine dose against diphtheria and tetanus, we determined the immunity levels in adults to identify most susceptible groups for protection in Singapore. METHODS: Our study involved residual sera from 3293 adults aged 18-79 who had participated in a national health survey in 2010. IgG antibody levels were determined using commercial enzyme-linked immunosorbent assay. RESULTS: Overall, 92.0% (95% confidence interval [CI]: 91.1-92.9%) had at least basic protection against diphtheria (antibody levels ≥0.01 IU/ml), while 71.4% (95% CI: 69.8-72.9%) had at least short-term protection against tetanus (antibody levels >0.1 IU/ml). The seroprevalence declined significantly with age for both diseases; the drop was most marked in the 50- to 59-year age group for diphtheria and 60- to 69-year age group for tetanus. There was a significant difference in seroprevalence by residency for diphtheria (92.8% among Singapore citizens versus 87.1% among permanent residents; P = 0.001). The seroprevalence for tetanus was significantly higher among males (83.2%) than females (62.4%) (P < 0.0005). CONCLUSIONS: It may be of value to consider additional vaccination efforts to protect older adults at higher risk for exposure against diphtheria and tetanus, particularly those travelling to areas where diphtheria is endemic or epidemic.