BACKGROUND: Identifying local outbreaks and their drivers is a key step towards curbing HIV transmission and potentially achieving HIV elimination. Such outbreaks can be identified as transmission clusters extracted from phylogenetic trees constructed of densely sampled viral sequences. In this study, we combined phylogenetic transmission clusters with extensive data on virological suppression and behavioral risk of cluster members to quantify the drivers of ongoing transmission over ten years.
METHODS: Using the comprehensive Swiss HIV Cohort Study and its drug-resistance database, we reconstructed phylogenetic trees for each year between 2007-2017. We identified HIV transmission clusters dominated by men who have sex with men(MSM) and determined their annual growth. We used Poisson regression to assess if cluster-growth was associated with a per-cluster-infectivity and behavioral-risk score.
RESULTS: Both infectivity and behavioral risk scores were significantly higher in growing MSM transmission clusters compared to non-growing clusters (p≤0.01). The fraction of transmission clusters without infectious members acquiring new infections increased significantly over the study period. The infectivity score was significantly associated with per-capita incidence of MSM transmission clusters in eight years, while the behavioral risk score was significantly associated with per-capita incidence of MSM transmission clusters in three years.
CONCLUSIONS: We present a phylogenetic method to identify hotspots of ongoing transmission among MSM. Our results demonstrate the effectiveness of treatment as prevention at the population level. However, the significantly increasing number of new infections among transmission clusters without infectious members highlight a relative shift from diagnosed to undiagnosed individuals as drivers of HIV transmission in Swiss MSM.