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Most Large HIV Molecular Clusters in US Primarily Involve MSM
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2022 CROI, February 12-16 and 22-24, 2022
Mark Mascolini
Although large and quickly growing HIV molecular clusters of people who inject drugs (PWID) in the United States garnered a large share of recent scientific and media attention-and the biggest public health responses-a new nationwide analysis by the Centers for Disease Control and Prevention (CDC) found that over 80% of large HIV clusters mainly involve men who have sex with men (MSM) [1]. These big MSM clusters share worrying traits, the CDC said: a transmission rate 6 times the overall national rate and fast growth rates within the cluster.
Analysis of HIV-related molecular data can uncover clusters not detected by other methods, the CDC noted, while spotting rapid transmission within clusters. By detecting and better understanding molecular HIV clusters, CDC investigators hope to help guide planning of public health responses.
The key molecular data involved in this cluster study come from analysis of HIV pol sequences reported to the National HIV Surveillance System for people diagnosed with HIV in the prior 3 years. The CDC study focused on clusters first detected in 2018-2019 and tracked through September 2021. The researchers inferred clusters with a pairwise threshold of 0.005 substitutions per genetic site. They defined rapid transmission clusters as those with 5 or more new diagnoses during the most recent 12 months; large clusters were those with more than 25 people. When more than 50% of an HIV transmission risk group (such as MSM or PWID) made up a cluster, they called that cluster primarily dominated by the majority risk group. The CDC team used molecular clock phylogenetic analysis to estimate transmission rate within a cluster. Annual growth rate compared cluster size at detection to size in September 2021.
The analysis began with 144 HIV molecular clusters first identified in 2018-2019. MSM dominated a large majority of those clusters-118 or 82%-while PWID dominated 12 clusters or 8%, and no primary risk group could be identified for the remaining 14 clusters. The investigators counted 32 large clusters (22% of 144), 23 of them (72%) primarily dominated by MSM, 6 (19%) dominated by PWID, and 3 (9%) with no primary risk group.
Large MSM clusters grew swiftly from a median size of 10 people at cluster detection to 34 people by September 2021. Large MSM clusters had brisk transmission rates, averaging 23 transmissions per 100 person-years, almost 6 times the overall US transmission rate of 4 transmissions per 100 person years. Perhaps most contrary to expectation, large MSM clusters had broad racial/ethnic diversity-31% black, 31% Hispanic, 30% white, and 9% other.
While large MSM clusters consisted mainly of cisgender men (91%), these clusters also included cisgender women (4%), transgender women (4%), and transgender men (1%). The CDC believes these transgender proportions are underestimates. Sex between men accounted for 76% of HIV cases in large MSM clusters, distantly followed by sex between men plus injection drug use (5%), high-risk heterosexual sex (4%), injection drug use alone (3%), and “other” (13%).
Suggesting shifting geographic trends in the US HIV epidemic, most large MSM clusters lived in the South (10, 40%), closely followed by the West (9, 36%), and trailed by the Northeast (3, 12%) and Midwest (3, 12%).
The CDC team proposed that small initial cluster sizes coupled with fast growth rates for HIV clusters in the United States indicate that “swift response when clusters are first detected has the potential for meaningful public health impact.” They suggested that HIV transmission via clusters can be stopped only by interventions based on the unique needs of specific clusters.
Reference
1. Perez S, Panneer N, France AM, Curran K, Oster AM. Large HIV clusters among men who have sex with men in the United States. 2022 CROI, February 12-16 and 22-24, 2022. Abstract 44.
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