Molecular epidemiological characteristics of HIV-1 among young men who have sex with men (MSM) in Hangzhou, 2022–2024

Objective To characterize HIV-1 subtype distribution, transmission clustering, and associated demographic and temporal risk factors among newly diagnosed young men who have sex with men (MSM) aged <30 years in Hangzhou (2022–2024), comparing student and non-student subgroups. Methods Pol sequences from 857 newly diagnosed, ART-naive young MSM underwent phylogenetic, molecular transmission network, and Bayesian evolutionary analyses; intergroup differences were assessed with chi-square or Fisher’s exact tests and logistic regression. Results Twelve subtypes/CRFs were identified, with CRF07_BC (46.67%) and CRF01_AE (35.82%) dominant. At a 1.0% genetic distance, 249 of 857 cases (29.05%) formed 81 clusters; 62 suspected high-transmission-risk cases were predominantly CRF01_AE (54.84%). Both largest clusters were CRF01_AE, with posterior median Re above 1 in 2024 but wide 95% HPDs that included values below 1. Clustering risk was higher among cases registered elsewhere in Zhejiang Province (OR = 1.523, 95% CI: 1.041–2.230) and in the 20–24 years group (OR = 1.501, 95% CI: 1.094–2.061). This age group was also more likely to be classified as suspected high-transmission-risk cases (OR = 2.994, 95% CI: 1.483–6.047), as were cases with CRF01_AE infection relative to others/URFs (OR = 3.471, 95% CI: 1.271–9.479). Household registration in other cities of Zhejiang Province was associated with increased odds of others/URFs relative to CRF01_AE infection (OR = 1.837, 95% CI: 1.130–2.986, p = 0.014), compared with household registration outside Zhejiang Province. Student status was not observed to be associated with clustering or high-transmission-risk classification. Conclusion Hangzhou’s young-MSM epidemic is genetically diverse; two CRF01_AE clusters sustained transmission risk (posterior median Re > 1 in 2024) after 3–6 years of local circulation. Prevention should target the broader young MSM population, not students alone.