Using routine genetic analyses of viral strains seen among those newly diagnosed with HIV, the Centers for Disease Control and Prevention (CDC) has identified scores of rapidly expanding transmission clusters nationwide. These clusters, in which HIV is apparently spreading quickly among sexual networks, disproportionately affect young men who have sex with men (MSM), in particular Latinos.

According to a CDC analysis released in 2017, the HIV infection rate among Latino MSM has increased in recent years while it has decreased among Black and white MSM.

CDC researcher Anne Marie France, PhD, presented findings from the new study on transmission clusters at the 2018 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston.

The CDC’s Anne Marie France speaks at CROI 2018 in Boston.

The CDC’s Anne Marie France speaks at CROI 2018 in Boston.Benjamin Ryan

During a 12-month period beginning December 31, 2015, CDC investigators conducted genetic analyses on the HIV of 51,750 people who were diagnosed with the virus during the previous three years.

Based on genetic similarities seen among the viral strains, the investigators identified 60 rapidly growing transmission clusters. (A cluster was defined as at least five linked viral strains from diagnoses made during a 12-month period.) These clusters, which were located in all regions of the country and in 20 states, included up to 42 people with linked virus.

The members of these clusters contracted HIV at a very high rate: a median 44 transmissions per 100 cumulative years of life, with a range across the clusters of 21 to 132 transmissions per 100 cumulative years of life. To determine the denominator in this equation (the cumulative years of life), the researchers used established methods, specifically a tool called molecular phylogenetic analysis, to estimate when diagnosed individuals had been infected and then extrapolated accordingly about the cumulative years of life that applied to the members of each cluster.

Compared with the 50,847 newly diagnosed individuals not identified as members of rapidly growing clusters, the 903 people in the transmission clusters were disproportionately young MSM (a respective 31 percent versus 62 percent of those inside and outside a transmission cluster were young MSM) and in particular young Latino MSM (10 percent versus 26 percent).

Referring to the technology that enabled the genetic analyses conducted in the study, France said, “While we’re early in the implementation process, we believe this technology holds tremendous promise in helping identify transmission networks of potential concern. And we’ll be continuing to work with state and local communities to further assess and intensify interventions as needed to slow the HIV epidemic in the U.S.”