Unravelling dengue serotype 3 transmission in Brazil: evidence for multiple introductions of the 3III_B.3.2 lineage.

Dengue, caused by dengue virus (DENV) 1-4, remains a global public health concern, with Brazil experiencing some of the largest epidemics. The re-emergence of DENV-3 in Brazil between 2023 and 2024 has raised concerns about new outbreaks due to the absence of sustained circulation of this serotype in recent years. This study investigates the dynamics of DENV-3 in Brazil, focusing on the spread of the 3III_B.3.2 lineage within genotype 3III and its introduction routes. We analysed 1536 DENV-3 genomes, including 11 newly generated in this study, all classified as genotype 3III, the dominant DENV-3 genotype in Brazil since 2001. Phylogenetic analysis identified the 3III_B.3.2 lineage in all recent Brazilian cases, with detections also reported in Central America, the USA, and Europe. At least six independent introduction events of this lineage into Brazil were identified, with the Caribbean region as the primary source. The earliest introduction likely occurred in late 2022 in Roraima, followed by introductions in São Paulo, Minas Gerais, and Pará. While one instance of interstate transmission was detected-from São Paulo to Minas Gerais-our findings indicate that external introductions, rather than domestic spread, were the primary drivers of DENV-3 circulation during this period. These results underscore the importance of continued genomic surveillance and coordinated public health strategies to monitor and mitigate future outbreaks.