Molecular investigation of a new HIV-1 outbreak among people who inject drugs in Greece: evidence for a dense network of HIV-1 transmission

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Objectives Αn HIV-1 outbreak was identified among people who inject drugs (PWID) in Thessaloniki, Greece, during 2019–2021. We aimed to investigate the characteristics of this outbreak by means of molecular epidemiology. Methods We analysed 57 sequences from PWID sampled in Thessaloniki during 2019–2023. Phylogenetic trees were inferred using all subtype A sequences from PWID sampled since 1999 in Greece and reference sequences (n=4824). Phylodynamic analysis was performed using the Bayesian birth-death skyline serial model. Results Most of the 57 study sequences belonged to sub-subtypes A6 (49, 86%) and A1 (4, 7%). Phylogenetic analysis revealed that two (50%) A1 sequences clustered together and 47 (95.9%) A6 sequences fell within three PWID-specific phylogenetic clusters. The 99.6% and 77.9% of pairwise genetic distances within the largest and second largest PWID clusters were lower than 0.015 substitutions/site. Using a more stringent threshold (0.0015 substitutions/site), we identified five networks of sequences from PWID infected within 1 year. The effective reproduction number (R e ) started to increase at the beginning of 2019 and remained high almost until the end of 2021. The estimated time from HIV-1 infection to diagnosis showed an increasing trend during 2020–2023 (p<0.001). Conclusions The regional clustering of the PWID sequences and their low genetic divergence confirm its local spreading and the recent nature of the outbreak. Using a stringent genetic distance threshold, we showed that HIV-1 transmission occurred among large groups of PWID. The time of epidemic growth coincided with the time of the initial identification, and HIV-1 transmission continued at high rates until 2021.

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