The evaluation of anti-diphtheria toxoid antibodies in healthy population in Kon Tum, Vietnam: a population-based study

Background: Despite diphtheria immunization are to apply an effective primary immunization in childhood and to maintain immunity throughout life. Cases of diphtheria have been reported in Viet Nam in recent years. The aim of this study was to evaluate the seroprevalence of IgG antibodies to diphtheria toxoid among healthy person population in Kon Tum, Viet Nam. Methods: Blood samples were obtained from 2225 healthy persons aged 2-98 years collected in 2019 and 2020. Samples were tested for diphtheria toxoid antibodies by commercial Anti-Diphtheria Toxoid IgG Enzyme-Linked Immunosorbent Assay (ELISA). Results: An antibody level of < 0.01 IU/mL (susceptibility) was found in 802 (36.0%) of the 2225 subjects, 136 (6.1%) had antibody levels of 0.01-0.099 IU/mL (basic protection), and 1287 (57.8%) had antibody levels >= 0.1 IU/mL (full protection). The full protection level increased significantly in persons aged above 60 years with antibody levels of 70.6%. No significant difference in seroprotection prevalence was found according to gen -der, ethnicity, residence, education and occupation. The results also demonstrated that people with vaccination against diphtheria during past 10 years were found to have a high immunity (83.8%) compared to 54.8% (OR: 4.7; 95%CI: 3.8-6.5) and 60.7% (OR: 3.8; 95%CI: 2.6-5.7) in persons with no and unknown vaccination (p < 0.0001). Conclusions: The level of anti-diphtheria toxoid antibodies among children and adults in Kon Tum was low. The high risk of diphtheria outbreaks may occur among individuals lacking basic immunity against diphtheria.