BACKGROUND: Clusters of rapid HIV transmission indicate larger underlying networks that are not effectively reached by HIV prevention, testing, and care services. Starting in 2018, the Centers for Disease Control and Prevention (CDC) funded 59 U.S. health departments (HDs) to detect and respond to HIV clusters; HDs began reporting clusters to CDC in January 2020. METHODS: For clusters reported to CDC, we described cluster characteristics at detection, including detection method; size; HIV transmission category, defined as that of >50% of cluster members; and HD investigation and response activities. RESULTS: During 2020-2022, 45 HDs reported 322 HIV clusters, with most detected by molecular analysis of HIV sequences (75%). Most were detected in the South (46%) and three-quarters were predominant sexual transmission. Median cluster size at detection for molecular clusters was 10 persons (interquartile range 7-18). Among 205 clusters with follow-up data, investigation and response activities were conducted for 95%, including direct outreach to persons in clusters for partner services (64%), medical chart reviews (42%), and focused testing events (13%). Limited data on named partners tested showed that 11% received new HIV diagnoses. CONCLUSIONS: HD HIV cluster detection activities detected many clusters. Response activities were tailored for different clusters and intervened in networks with rapid transmission and high undiagnosed infection, as indicated by high positivity among partners. Cluster detection and response is an important tool to identify and address gaps in HIV prevention, testing, and care that facilitate rapid transmission.

OBJECTIVE: To estimate the influence of bursts of rapid HIV transmission on future transmission and describe populations affected by transmission bursts. DESIGN: Phylogenetic analysis of US National HIV Surveillance System data. METHODS: Time-scaled phylogenetic trees were inferred for six geographic regions using sequences from persons with HIV (PWH) with diagnoses of HIV infection 2014-2019. Transmission bursts were defined as ≥3 adjacent inferred transmission events in the phylogeny during a detection period. We calculated the relative contribution of transmission bursts 2015-2016 to transmission 2017-2019 compared with non-bursts. Then, we detected bursts within any sliding 2-year period 2014-2019 and assessed descriptive associations of characteristics of individuals involved with or descended from transmission bursts using univariate risk ratios. RESULTS: The 5.6% of phylogenetic lineages involved in transmission bursts 2015-2016 contributed to 14.9% of inferred transmission events 2017-2019. The relative contribution of lineages involved in transmission bursts to future transmission was 2.94 times that of lineages not involved in bursts. Younger age at diagnosis, self-identification as transgender or an additional gender identity, or as a cisgender man, male-to-male sexual contact, injection drug use, or male-to-male sexual contact and injection drug use, and diagnosis during acute or early infection were most strongly associated with involvement in or descendance from transmission bursts. CONCLUSIONS: Transmission bursts contribute disproportionately to future HIV transmission, underscoring the value of detecting and responding to rapid transmission to reduce incidence. Bursts of rapid transmission may also contribute to enduring disparities in incidence among some key populations.