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US HIV Transmission Network Study Holds
Some Surprises About Women and MSM
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CROI 2014, March 3-6, 2014, Boston
Mark Mascolini
Nationwide construction of an HIV transmission network by the Centers for Disease Control and Prevention (CDC) confirmed some inferences about how HIV gets passed along in the United States--but it also held a few surprises [1]. Most notably, the transmission network revealed that heterosexual women--at least those in this big analysis--have sexual links to men who have sex with men (MSM) more than to heterosexual men.
Researchers can infer HIV transmission networks by putting viral sequences side by side and seeing if they match closely enough to indicate that one person passed HIV to the other. Networks usually get built on a local scale, but CDC researchers decided to tap US National HIV Surveillance System (NHSS) data from 2001-2012 to compare sequences--two at a time--across the country.
This vast enterprise involved 41,294 HIV-1 sequences sampled from people at least 13 years old. Of the 41,294 people represented, 32,276 (78%) were men and 9018 women. Almost half (47%) were black, 26% were white, and 22% were Hispanic. Transmission categories were male-to-male sex for 61%, females with heterosexual contact for 18%, males with heterosexual contact for 8%, male injection drug user (IDU) for 5%, female IDU for 4%, and male-to-male sex plus injection drug use for 4%.
Almost one third of all sequences analyzed, 12,910 or 31%, fit into a transmission cluster with one or more partners. These 12,910 individuals, or nodes, linked with partners to create 29,493 pairs. Although MSM accounted for 61% of sequences, they made up 76% of the final network. In almost all transmission pairs involving MSM, the partner was also an MSM (82%) or an MSM/IDU (7%).
But every other risk group--heterosexual women and men, and male and female IDUs--also had sexual contacts with MSM. In fact, heterosexual women proved more likely to be linked to MSM (44%) than to heterosexual men (18%) or IDUs (13%). MSM/IDUs linked to MSM (88%) far more often than to IDUs (2%).
The network showed that blacks most often had sex with other blacks (63%), while whites formed pairs with other whites less often (47%) and Hispanics with Hispanics even less frequently (27%). Hispanics formed pairs more often with whites (45%) than with other Hispanics. Whites paired fairly often with Hispanics (27%) and with blacks (19%).
Four out of 5 pairs (82%) consisted of people living in the same state at HIV diagnosis. Same-state pairs were most common among heterosexual women (91%) and female IDUs (90%) and least common among MSM (81%) and MSM IDUs (81%).
The CDC team concluded that racial/ethnic and geographic mixing in HIV transmission is far from rare, though mixing frequency varies by population. Since MSM and all other risk groups had links with MSM, the researchers suggested "interventions that reduce transmissions by MSM are likely to reduce HIV acquisition among other risk groups as well." That may be particularly true in the Midwest and West, they observed, where most pairs including heterosexual women had links to MSM.
Reference
1. Oster AM, Wertheim JO, Hernandez Al, et al. HIV transmission in the United States: the roles of risk group, race/ethnicity, and .geography. CROI 2014. Conference on Retroviruses and Opportunistic Infections. March 3-6, 2014. Boston. Abstract 213.
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