Molecular analysis allows inference into HIV transmission among young men who have sex with men in the United States.

OBJECTIVE: The objective of this study is to understand the spread of HIV among and between age and racial/ethnic groups of men who engage in male-to-male sexual contact (MSM) in the United States.
DESIGN: An analysis of HIV-1 pol sequences for MSM collected through the US National HIV Surveillance System (NHSS) during 2001-2012.
METHODS: Pairwise genetic distance was calculated to determine potential transmission partners (those with very closely related nucleotide sequences, i.e. distance ≤1.5%). We described race/ethnicity and age of potential transmission partners of MSM.
RESULTS: Of 23 048 MSM with HIV sequences submitted to NHSS during 2000-2012, we identified potential transmission partners for 8880 (39%). Most potential transmission partners were of the same race/ethnicity (78% for blacks/African-Americans, 64% for whites and 49% for Hispanics/Latinos). This assortative mixing was even more pronounced in the youngest age groups. Significantly fewer young black/African-American and Hispanic/Latino MSM had older potential transmission partners than young white MSM.
CONCLUSION: Black/African-American MSM, who are more profoundly affected by HIV, were more likely to have potential HIV transmission partners who were of the same race/ethnicity and similar in age, suggesting that disparities in HIV infections are in large part not due to age-disassortative relationships. Concerted efforts to increase access to preexposure prophylaxis, quality HIV care and effective treatment are needed to interrupt transmission chains among young, black/African-American MSM.