Social network-based recruitment successfully reveals HIV-1 transmission networks among high-risk individuals in El Salvador

Abstract

Objective: HIV in Central America is concentrated among certain groups such as men who have sex with men (MSM) and female sex workers (FSWs). We compared social recruitment chains and HIV transmission clusters from 699 MSM and 787 FSWs to better understand factors contributing to ongoing HIV transmission in El Salvador.

Methods: Phylogenies were reconstructed using pol sequences from 119 HIV-positive individuals recruited by respondent-driven sampling (RDS) and compared with RDS chains in 3 cities in El Salvador. Transmission clusters with a mean pairwise genetic distance ≤ 0.015 and Bayesian posterior probabilities =1 were identified. Factors associated with cluster membership were evaluated among MSM.

Results: Sequences from 34 (43%) MSM and 4 (10%) FSW grouped in 14 transmission clusters. Clusters were defined by risk group (12 MSM clusters) and geographic residence (only 1 spanned separate cities). In 4 MSM clusters (all n = 2), individuals were also members of the same RDS chain, but only 2 had members directly linked through recruitment. All large clusters (n ≥ 3) spanned >1 RDS chain. Among MSM, factors independently associated with cluster membership included recent infection by BED assay (P = 0.02), sex with stable male partners (P = 0.02), and sex with ≥ 3 male partners in the past year (P = 0.04).

Conclusions: We found few HIV transmissions corresponding directly with the social recruitment. However, we identified clustering in nearly one-half of MSM suggesting that RDS recruitment was indirectly but successfully uncovering transmission networks, particularly among recent infections. Interrogating RDS chains with phylogenetic analyses may help refine methods for identifying transmission clusters.

Figure 1. Bayesian phylogenetic tree based on pol sequences from MSM (n=80) and FSW (n=39, shown with black dots) who were identified with HIV-1 infection during respondent-driven sampling (RDS) recruitment in 2008

Transmission clusters were identified with a mean intra-cluster pairwise genetic distance ≤0.015 nucleotide substitutions per site and were confirmed with Bayesian posterior probability=1 (shaded circles labeled A-N). Colored circles correspond to individuals involved in the example RDS chain shown in Figure 2. Nodes with posterior probabilities >0.90 are indicated. Control sequences from the Los Alamos HIV database are labeled by accession number.

Figure 2

Respondent-driven sampling (RDS) chain demonstrating the most successful chain at identifying phylogenetic transmission clusters. The RDS chain started from one MSM seed in San Salvador and resulted in the recruitment of 134 men. Arrows indicate the direction of recruitment. Eleven men belonged to six discrete clusters (noted by colored circles which also correspond to the phylogenetic clusters indicated in Figure 1. However, none of these men were directly linked in the RDS chain. *Indicates three HIV-positive participants without sequences.