Phylogenetic analysis and transmission networks highlight the role of older adults in the transmission of HIV-1 in northern Zhejiang, China.

Background: In China, human immunodeficiency virus (HIV) poses a significant challenge to older heterosexual adults. This study explored the transmission dynamics of HIV infection among older adults in northern Zhejiang Province, China using phylogenetic analysis and transmission networks.

Methods: HIV pol sequences without any antiretroviral therapy were obtained from newly diagnosed HIV-positive patients in Huzhou between 2017 and 2022. Pairwise genetic distances between sequences were calculated using HIV Trace based on the Tamura-Nei 93 method. The transmission network was constructed using Cytoscape v3.9.1. The effective reproductive number (Re) of each large cluster was estimated using the birth-death skyline model in BEAST v2.4.2. Multivariate logistic regression was performed to identify factors associated with clustering using R v4.4.3 software.

Results: A total of 931 HIV pol sequences were successfully obtained, of which CRF07_BC (51.7%, 481/931) and CRF01_AE (27.5%, 256/931) were the predominant subtypes. The proportion of CRF07_BC increased from 43.5% in 2017 to 59.8% in 2022, whereas that of CRF01_AE decreased from 33.3% in 2017 to 19.5% in 2022. In total, 448 individuals formed 110 putative transmission networks with a clustering rate of 48.1%, ranging from 2 to 83 sequences per network. Four large clusters were identified, with a higher proportion of individuals aged ≥ 50 years (49.0%) compared to that in small/ medium clusters (35.5%) and non-clustered cases (26.1%). Multivariable logistic regression showed that clustering was associated with age ≥ 50 years (adjusted odds ratio [aOR] = 2.125, 95% confidence interval [CI]: 1.251-3.632), registered households in Huzhou (aOR: 1.677, 95% CI: 1.252-2.249), and CRF07_BC subtype (aOR: 2.119, 95% CI: 1.542-2.924). Only one of the four large transmission clusters had a Re > 1, with a high proportion (63.0%, 29/46) of male older adults exposed through commercial sexual contact.

Conclusions: The subtype of CRF07_BC was the predominant subtype locally, showing an increasing trend over time. Molecular transmission network analysis and multivariate logistic regression revealed that older adults play a key role in local HIV-1 transmission. Public health services should target this key population to curb the spread of HIV-1.