Co-circulation of three dengue virus serotypes led to a severe dengue outbreak in Xishuangbanna, a border area of China, Myanmar, and Laos, in 2019

•A large-scale dengue fever outbreak occurred in the China, Myanmar, and Laos border area in 2019.•Three dengue virus (DENV) serotypes were found to be co-circulating, which is very rare in this area.•DENV-1 and DENV-2 were the main pathogenic agents of this outbreak.•The co-circulation may have led to more serious clinical symptoms and a larger scale epidemic. Dengue fever was included in the top 10 global health threats announced by the World Health Organization (WHO) in early 2019. In some southern provinces of China, autochthonous outbreaks have also been reported over the last decade. An unexpected large outbreak of dengue fever was reported in Xishuangbanna, a border area of China, Myanmar, and Laos, in 2019. Among the 226 hospitalized cases, 90 were diagnosed as severe dengue according to the 2009 WHO guidelines. Serotyping and phylogenetic analyses of envelope gene sequences from 246 randomly selected samples showed that three serotypes of dengue virus were co-circulating in this outbreak, which is very rare in this area. Dengue virus serotype 1 (DENV-1, genotype I) and serotype 2 (DENV-2, Cosmopolitan genotype and Asian genotype) were the main pathogenic agents of this outbreak. Dengue virus serotype 3 (DENV-3) epidemic strains were classified as genotype III and formed a close cluster with the Thailand 2015 epidemic strain. The co-circulation may have led to more serious clinical symptoms and a larger scale epidemic. This finding is of great importance in understanding the circulation of DENV and to strengthen the detection and management of dengue fever in border areas.