A study on the sources of HIV-1 transmission in Haikou, China: insights from molecular genetics and field epidemiological investigations

Objective This study aimed to integrate molecular network analysis, Bayesian phylogenetic modeling, and field epidemiological investigation to systematically trace the sources of HIV-1 transmission in Haikou, China. Design A longitudinal molecular surveillance program was implemented between 2021 and 2024, combined with a cross-sectional field epidemiological investigation. Methods Subtype-specific local sequence databases were constructed from nationwide sequences. Molecular clusters containing both local and external sequences were subjected to phylogeographic analysis under a Bayesian relaxed molecular clock and a Skygrid model to infer transmission origins. Complementing the molecular analyses, field epidemiological investigations were carried out among a subset of participants to assess self-reported transmission sources. Results Among the 1,125 pol sequences, CRF01_AE (35.64%), CRF07_BC (38.58%), CRF65_cpx (7.20%), and CRF55_01B (4.53%) were the four most prevalent subtypes, with local clustering rates of 36.65%, 33.64%, 86.42%, and 54.90%, respectively. Phylogeographic analysis identified Guangdong as the primary origin of HIV-1 introductions into Haikou for all four subtypes, followed by Beijing (CRF01_AE_C1 and CRF65_cpx) and Shanghai (CRF07_BC). CRF01_AE showed bidirectional transmission between Haikou and Sanya, and between Haikou and Dongfang; CRF07_BC and CRF65_cpx both disseminated from Haikou to six other cities in Hainan. Field epidemiological investigations revealed that 52.33% (236/451) of participants had acquired HIV outside Haikou, with Guangdong being the primary source (16.95%, 40/236), followed by Sichuan (5.93%, 14/236). Within Hainan, Sanya accounted for the largest share (7.63%, 18/236). Conclusion By integrating molecular and epidemiological evidence, this study advances the tracing of HIV-1 transmission sources and underscores the critical need for cross-regional prevention strategies targeting mobile populations.