Seroepidemiology and Carriage of Diphtheria in Epidemic-Prone Area and Implications for Vaccination Policy, Vietnam

Abstract

In 2019, a community-based, cross-sectional carriage survey and a seroprevalence survey of 1,216 persons 1-55 years of age were conducted in rural Vietnam to investigate the mechanism of diphtheria outbreaks. Seroprevalence was further compared with that of an urban area that had no cases reported for the past decade. Carriage prevalence was 1.4%. The highest prevalence, 4.5%, was observed for children 1-5 years of age. Twenty-seven asymptomatic Coerynebacterium diphtheriae carriers were identified; 9 carriers had tox gene-bearing strains, and 3 had nontoxigenic tox gene-bearing strains. Child malnutrition was associated with low levels of diphtheria toxoid IgG, which might have subsequently increased child carriage prevalence. Different immunity patterns in the 2 populations suggested that the low immunity among children caused by low vaccination coverage increased transmission, resulting in symptomatic infections at school-going age, when vaccine-induced immunity waned most. A school-entry booster dose and improved infant vaccination coverage are recommended to control transmissions.

Keywords: Corynebacterium species; Vietnam; bacteria; carriage; diphtheria; disease outbreaks; epidemic-prone area; immunity; seroepidemiology; vaccination policy; vaccine.

Figure 1

Study areas and locations where the cases were identified before and during diphtheria study in Tay Tra and Son Ha districts in Quang Ngai Province, Vietnam. Red and purple indicate 10 communes selected for this study. Blue and purple indicate 1 laboratory-confirmed diphtheria case reported during January‒September 2019 in each of these communes. Purple (Son Ha commune) indicates 12 confirmed cases reported in this commune within 1 month from the survey date, October 2019. Green indicates 2 communes excluded from the selection process of this study because a mop-up vaccine campaign was conducted in 2018. Inset map shows location of the study area in Vietnam.

Figure 2

Best-fitted linear regression line (blue line) comparing log-transformed IgG concentrations measured by Binding Site and IBL ELISAs. Shaded region indicates 95% CI.

Figure 3

Comparison of the age-stratified seroprevalence, the proportion of persons who had diphtheria toxoid antibody >0.1 IU/mL, between Quang Ngai Province and Nha Trang City (15), Vietnam. Seroprevalence of Quang Ngai was not weighted by population for this comparison. Nha Trang is a well-vaccinated community that has had no reported diphtheria cases since 2013. Error bars indicate 95% CIs.