One Quarter of HIV Infections in US Group Form Clusters or Networks

One quarter of HIV sequences from almost 4000 people in a US study group were genetically related—meaning they involved people who probably infected each other. One network included related viral sequences from 336 people.

The pol region of the HIV gene can be analyzed to detect resistance mutations. Similarities in pol gene sequences also show which individual HIVs are closely related and so may indicate transmission from one person to another.

Researchers analyzed pol gene sequences sampled from 3697 people in the CFAR Network of Integrated Clinical Systems (CNICS) cohort from 2000 to 2009 to determine which were genetically related. They analyzed only the first available sequence from each cohort member included. The CNICS team defined transmission clusters as 2 or more sequences with 1.5% or less genetic distance between them. They defined networks as clusters including 3 or more individuals.

Of the 3697 HIV pol sequences analyzed, 24% fell into clusters, including 155 clusters of 2 individuals, 54 clusters including 3 to 14 individuals (networks), and 1 huge network including 336 individuals.

Multivariate statistical analysis identified five factors associated with being in a cluster: not using antiretrovirals at the time of viral sampling (P < 0.001), viral sequence collected after 2004 (P < 0.001), CD4 count above 350 cells/µL (P < 0.01), viral load 10,000 to 100,000 copies/mL (P < 0.001), and viral load above 100,000 copies/mL (P < 0.001).

HIV sequences from women were more likely to cluster with those of other women (P < 0.001), and sequences from African Americans were more likely to cluster with sequences of other African Americans (P < 0.001).

The association between clustering and lack of antiretroviral therapy and higher viral load, the researchers suggest, implies that “transmission may be interrupted by earlier [HIV] diagnosis and treatment.” The observed female and African-American networks “reinforce the importance of diagnosis and prevention efforts targeted by sex and race.”

Source: Jeannette L. Aldous, Sergei Kosakovsky Pond, Art Poon, Sonia Jain, Huifang Qin, James S. Kahn, Mari Kitahata, Benigno Rodriguez, Ann M. Dennis, Stephen L. Boswell, Richard Haubrich, Davey M. Smith. Characterizing HIV transmission networks across the United States. Clinical Infectious Diseases. 2012; 55: 1135-1143.

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Written by Mark Mascolini on behalf of the International AIDS Society