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.