Contribution of HIV transmission bursts to future HIV infections, United States

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.