Diphtheria remains a public health concern in Vietnam. This study used whole-genome sequencing to characterize the genetic diversity, virulence, and antimicrobial resistance profiles of 21 Corynebacterium diphtheriae isolates collected from 2013 to 2024. Phylogenetic relationships were determined using core-genome MLST (cgMLST) and SNP analysis. AMR was assessed via phenotypic testing (CLSI/EUCAST standards) and genotypic screening (staramr/ARIBA/ResFinder/diphtOscan). Vaccination histories were categorized based on the Vietnam National Expanded Program on Immunization schedule. Analysis revealed a novel sequence type (ST1040) in 19 isolates and ST244 in the remaining two isolates. The ST1040 lineage showed that the nearest neighbors were contemporary isolates from China and India (863–1030 allele differences), suggesting a regional East/Southeast Asian evolutionary context. In contrast, the ST244 lineage, isolated in Vietnam in 2014 and 2015, exhibited strong genetic relatedness to an Austrian ST244 strain (isolated in 2018), suggesting a potential earlier circulation or origin of this lineage in Vietnam, or a shared global reservoir. All 21 isolates were Gravis biovar, carried the tox gene, and were toxigenic. Sulfonamide (sul1) and tetracycline (tet(33) resistance genes were prevalent in the ST1040 lineage, with consistent genotype-phenotype correlation. Among the 15 cases with interpretable vaccination data, none had completed the four-dose routine schedule prior to exposure. In 12 cases with date-verified records, vaccination was confirmed as reactive (administered post-infection) rather than pre-exposure. To effectively control diphtheria, this study emphasises the emergence of novel and drug-resistant strains of C. diphtheriae in Vietnam, which calls for improved genomic surveillance, proactive vaccination programs, and ongoing AMR monitoring.
Hoàng et al. (Fri,) studied this question.