Objective:

To investigate the introduction, local transmission, geographic spread, and associated populations of HIV-1 circulating recombinant form CRF85_BC in Zhejiang Province, China, from 2017 to 2023.

Approach:

• Study design: Retrospective, population-based molecular epidemiological investigation using province-wide HIV-1 surveillance data from January 1, 2017, through December 31, 2023.
• Participants: The analytical cohort included 259 newly diagnosed individuals with CRF85_BC infection, complete core epidemiological data, and qualifying HIV-1 pol sequences.
• Laboratory methods: Sequences were generated using reverse transcription–polymerase chain reaction, nested polymerase chain reaction, and Sanger sequencing; subtype assignments underwent standardized verification.
• Analysis: Phylogenetic analysis, Bayesian phylogeography, and molecular transmission networks assessed origins, dissemination, and clustering.

Key Findings:

• CRF85_BC was detected in Zhejiang in 2017 and had reached all 11 prefecture-level cities by 2023, becoming the province’s fifth most prevalent HIV-1 subtype.
• Among 259 cases, 181 (69.88%) were male, 191 (73.75%) were aged 50 years or older, and 241 (93.05%) reported heterosexual transmission.
• Phylogeographic analysis indicated multiple introductions from Sichuan and Yunnan, followed by sustained transmission within Zhejiang centered on Jiaxing.
• At a genetic distance threshold of 0.008 substitutions per site, 183 individuals (70.66%) formed 41 molecular clusters. Jiaxing accounted for 48.63% of clustered cases; 58.60% of genetic linkages connected two Jiaxing cases.
• Adults aged 60 years or older had higher clustering in unadjusted analysis, but age was not independently associated with clustering after adjustment. Nonlocal household registration remained associated with lower clustering odds.

Interpretation:

The findings support repeated introductions followed by local establishment, with Jiaxing serving as a central transmission hub. Older adults’ higher clustering should be interpreted alongside residence and other population characteristics.

Limitations:

• Analysis was restricted to the pol region rather than full-length genomes.
• Undiagnosed infections and sequencing failures may have introduced selection bias and underestimated transmission linkages.
• Individual-level behavioral survey data were unavailable, limiting differentiation of heterosexual transmission risks.

Conclusion:

CRF85_BC became established across Zhejiang following repeated introductions from southwestern China. The findings support provincial molecular surveillance, cross-provincial monitoring, and targeted interventions focused on affected populations and core transmission clusters.

Sources:

• Spatiotemporal origin and cross-regional transmission dynamics of HIV-1 circulating recombinant form (CRF85_BC) in developed eastern China: a 7-year comprehensive molecular surveillance study