Background
Effectively identifying and responding to HIV molecular clusters, which indicate rapid HIV transmission, is an essential part of Ending the HIV Epidemic in the United States. We described the composition and growth of such HIV molecular clusters attributed to sexual transmission. Our aim was to identify characteristics of clusters with continued rapid growth to help communities prioritize those which may require the most robust and rapid response.
Methods
Using National HIV Surveillance System data reported through December 2023, we characterized molecular clusters first detected during December 2015–December 2022. We detected clusters quarterly, using transmission network analysis of HIV-1 pol sequences for all diagnoses in the past 3 years, using a pairwise threshold of 0.005 substitutions/site. Clusters of rapid transmission had ≥ 5 diagnoses during the past 12 months. Sexual transmission clusters are those with >50% of cluster members with transmission category of heterosexual or male-to-male sexual contact (MMSC). We compared characteristics of people in clusters with little/no growth to those in clusters with continued rapid growth (average annual growth <3 vs ≥3 diagnoses/year).
Results
Of 432 HIV molecular clusters, 386 (89%) were attributed to sexual transmission. Of 3,515 people in these clusters, most were men (92%), aged 20–29 years (54%), and resided in the South (53%). Transmission categories included MMSC (81%), other or no identified risk (7%), heterosexual transmission (6%), MMSC and injection drug use (IDU) (5%), and IDU (1%). Clusters varied in size at detection (median=8; interquartile range [IQR]: 6–11), annual growth (median=2; IQR: 1–5), and total growth (median=8; IQR: 3–18; range: 0–102). Clusters with continued rapid growth (n=159; 41%) had similar size at detection (median=9; IQR: 7–12) as clusters with little/no growth (n=227; median=8; IQR: 6–11), yet had higher annual growth (median=5; IQR: 4–8 vs. median=1; IQR: 1–2) and grew to include 10–110 persons. Age, transmission category, region, and early infection were similar across clusters (Table).
Conclusions
Sexual transmission clusters are common and vary in growth after detection; over one third had continued rapid growth. Our analysis did not identify predictors of cluster growth. Exploration of other factors (e.g., the level of response, HIV testing and its impact on cluster growth, and social determinants of health) might reveal important differences to further guide prioritization and response.