First Phylodynamic Analysis of HRSV-A and HRSV-B in Rio de Janeiro, Brazil, Reveals Recurrent Dispersals From the Megalopolis Center to Suburban Cities.

Human respiratory syncytial virus (HRSV) is the leading cause of acute lower respiratory tract infections in infants and represents a major burden to public health systems worldwide. Despite its epidemiological relevance, genomic surveillance of HRSV in Brazil remains limited. Here, we investigated the genetic diversity and dispersal dynamics of HRSV-A and HRSV-B in the state of Rio de Janeiro, Brazil, during the period immediately preceding the implementation of maternal vaccination and prophylactic monoclonal antibodies. A total of 136 near-complete HRSV genomes were recovered from respiratory samples collected between May 2024 and June 2025 using metagenomic sequencing. Phylogenetic analyses identified 11 HRSV-A and five HRSV-B lineages, with most sequences belonging to the A.D.3 and B.D.E.1 clades. Phylodynamic reconstructions revealed multiple independent introductions into the state, with the city of Rio de Janeiro acting as the primary within-state dispersal hub. Viral dispersal routes originating from the capital also showed greater mean transmission distances than those from other municipalities. Coinfections with additional respiratory viruses were frequently detected, and most HRSV-positive patients were children under 1 year of age. These results demonstrate that HRSV circulation in Rio de Janeiro is shaped by repeated introductions and strong spatial structuring centered around highly connected urban areas. Our findings underscore the importance of continuous genomic surveillance to monitor viral evolution, characterize transmission dynamics, and support science-based public health strategies against HRSV. Establishing robust monitoring systems will be critical to evaluate the impact of newly implemented immunization programs in Brazil.