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Over 400 million people are estimated to be at risk of acquiring dengue virus (DENV). Despite efforts to mitigate the impact of DENV epidemics, the virus remains a public health problem in the Americas: more than one million DENV cases were reported in the continent between January and July 2019 DENV was first detected in Brazil in 1982, and Brazil has reported 88% (1,127,244 cases) of all DENV cases in the Americas during 2019 to date. São Paulo state in the southeast of Brazil has reported nearly half of all DENV infections in the country. Here we characterised the genetic diversity of DENV strains circulating in São Paulo state in 2019, at the epicentre of the ongoing DENV epidemic. Using portable nanopore sequencing we generated 20 new DENV genome sequences from viremic patients with suspected dengue infection residing in two of the most-affected municipalities, Araraquara and São José do Rio Preto. We conducted a comprehensive phylogenetic analysis with 1,630 global DENV strains to better understand the evolutionary history of the DENV lineages that currently circulate in the region. The new outbreak strains were classified as DENV2 genotype III (American/Asian genotype). Notably, phylogenetic analysis indicated that the 2019 outbreak is the result of a novel DENV lineage that was recently introduced to Brazil from the Caribbean region. Our genetic analysis further indicates that the introduction and onwards spread of the outbreak lineage (named here DENV2-III BR-4) indicates a new DENV2 lineage replacement in Brazil. Dating phylogeographic analysis suggests that DENV2-III BR-4 was introduced to Brazil in or around early 2014, possibly from the Caribbean region. Our study describes the early detection of a newly introduced and rapidly-expanding DENV2 virus lineage in Brazil. <h4>Author Summary</h4> Dengue is the most important mosquito-borne viral disease of humans. The disease is caused by the dengue virus (DENV) that is classified within genus Flavivirus . DENV infections are caused by 4 serotypes (DENV 1-4) that are genetically related but antigenically distinct. Dengue infection results in a variety of symptoms that range from mild fever to dengue hemorrhagic fever and/or dengue shock syndrome (DHF/DSS). Clinical outcomes are associated with different types of infection, viral serotypes, genotypes, lineages, and host genetic factors. As a re-emerging infectious disease, DENV has become a serious threat to public health in the Americas, and particularly in Brazil, where it was introduced in the 1980s and became well established due to the country-wide re-infestation of the Aedes aegypti mosquito vector species. During the first six months of 2019, 1,282,183 DENV cases were reported in the Americas, with Brazil reporting a staggering 1,127,244 (88%) of all dengue cases in the continent. To date, no information exists on the genetic composition of the DENV lineage or lineages causing the current epidemic. Here we use portable sequencing to rapidly generate virus genome data from cases occurring in two different are severely-affected municipalities in São Paulo state, Brazil. We find that the 2019 dengue outbreak in Brazil is caused by a newly introduced DENV serotype 2 genotype III (Asian/American) that seems to be replacing previously-circulating DENV2 lineages. We discuss the potential implications of our results regarding the current outbreak in the context of previous outbreaks in the same region.

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