Abstract Very little is known about the HIV-1 epidemic in Fiji, nor the wider South Pacific region more generally, yet new reported HIV-1 infections are on the rise. As of 2023, there are an estimated 2000 cases of HIV-1 in Fiji with heterosexual contact the primary route of transmission. In this study, we used a molecular epidemiological approach to better understand the genetic diversity of the HIV-1 epidemic in Fiji and reveal patterns of viral transmission. Between 2020 and 2021, venous blood samples were collected from people who had previously been diagnosed with HIV-1. We generated molecular data from 53 infections, representing ~2–3% of reported cases, to identify HIV-1 subtypes and determine the outbreak’s trajectory. Among the 53 HIV-1 cases, we used Bayesian inference to estimate six separate introductions with at least two of these introductions leading to sustained transmission forming large, nation-wide clusters of HIV-1 subtype C. We found that since the introduction of public health interventions circa 2014, the effective reproductive number, R e , decreased among the major clusters identified from an average of 2.4 to just below 1. Molecular epidemiological analysis suggested that public health efforts aimed at decreasing the spread of the disease were at least somewhat effective. Nevertheless, with a recent rise in reported HIV-1 cases, this study demonstrates the utility of molecular data to inform a more targeted public health approach for controlling its spread.
