Molecular investigation of HIV-1 cross-group transmissions during an outbreak among people who inject drugs (2011-2014) in Athens, Greece.

New diagnoses of HIV-1 infection among people who inject drugs (PWID) rocketed in Athens, Greece between 2011 and 2014 (HIV-1 outbreak). Our aim was to identify, during that period, potential cross-group transmissions between the within-Greece PWID and other risk or national groups using molecular methods. Sequences from 33 PWID were outside the PWID-outbreak networks in Greece (PWID-imported transmissions). Phylogenetic analyses on 28 of these sequences (subtypes A and B) showed that 11 subtype B infections originated from Greece, whereas 8 and 7 subtype A strains were from former Soviet Union countries (A) and Greece, respectively. The putative source in half of the PWID-imported transmissions with Greek origin was an individual who acquired HIV via sexual contact. During four years of an HIV-1 outbreak among PWID in Athens, Greece, 33 individuals in this group (4.6% of all diagnoses with phylogenetic analyses) are likely to represent infections, sexually or injection-acquired, outside the within-Greece-PWID-outbreak networks. Combined molecular and traditional HIV surveillance to monitor introductions of new strains, and interventions that aim at reducing the rate of both injection and sexual risky practices are needed during drug injection-related HIV outbreaks.