Epidemiology and Phylogenomic Characterization of the Clade IIb C.1 Mpox Outbreak in Phnom Penh, Cambodia (2023-2024)

Mpox is an infectious disease caused by the Monkeypox virus, which is divided into two main genetic clades: Clade I and Clade II. A large-scale outbreak linked to Clade IIb emerged in 2022 and rapidly spread to more than 100 countries worldwide. Here, we describe the first and only documented Mpox outbreak in Cambodia (2023-2024), the public health outbreak response efforts and analysis, and integrating epidemiological and genomic approaches.To investigate the outbreak, samples from suspect cases were confirmed using qPCR before virus whole genome sequences were obtained for phylogenomic analyses.Epidemiological investigation revealed transmission primarily through intimate contact within socially connected networks, exclusively among men who have sex with men. None of the confirmed cases reported recent international travel or zoonotic exposure. Phylogenomic analysis showed that all Cambodian Mpox genomes belonged to lineage C.1, nested within Clade IIb. Bayesian analysis of publicly available C.1 genomes indicated that the most closely related sequence was from Thailand. Monophyletic clustering of Cambodian sequences, alongside a high proportion of APOBEC3 mutations, indicates localized human-to-human transmission after introduction.Altogether, these results illustrate the risk of regional lineages like C.1 introducing Mpox into previously unaffected countries, where socially connected human networks can sustain outbreaks despite control efforts.